Tech News Back Issues Issue: 041306

Deeper Mysteries of Graph2

By David Swain
Polymath Business Systems
www.polymath-bus-sys.com
dataguru@polymath-bus-sys.com

As database programmers, we are so often immersed in the minutia of storing and retrieving data that we sometimes forget the uses that our end users must make of that data and the information they need to glean from it. For example, most developers I have encountered over the years seem to address the issues of generating reports for an application only after the rest of that application has been built rather than making that ultimate use of the application the focus of their design. This practice only makes the reporting features of the application that much more difficult to realize.

Reports are one important way to help the end user filter out and make sense of momentarily significant collections of records. The generation of lists is another useful way of presenting the user with a focal set of data. But generating line upon line of printed or listed data is only the first step toward analyzing that data. The larger the data collection, the more difficult it is for the end user to grasp its meaning or to see subtle trends that can help them make intelligent plans for the future direction of their business.

Graphing is an essential part of any business-related database application. Converting masses of numerical data into a graphical form for analysis helps to highlight important information for the human decision-maker that might otherwise be hard to find - or be completely overlooked.

Consider the following set of data:

Date High Low Open Close
1-Feb-06
2.95
2.86
2.95
2.90
2-Feb-06
3.03
2.87
3.03
2.90
3-Feb-06
3.02
2.85
3.00
2.85
6-Feb-06
3.08
2.79
2.89
2.85
7-Feb-06
3.00
2.79
2.79
2.86
8-Feb-06
3.01
2.70
2.87
2.70
9-Feb-06
2.66
2.59
2.60
2.59
10-Feb-06
2.53
2.46
2.51
2.53
13-Feb-06
2.63
2.47
2.47
2.57
14-Feb-06
2.85
2.55
2.59
2.73
15-Feb-06
2.67
2.58
2.65
2.62
16-Feb-06
2.80
2.60
2.60
2.67
17-Feb-06
2.70
2.61
2.65
2.70
21-Feb-06
2.69
2.69
2.69
2.69
22-Feb-06
2.62
2.61
2.62
2.61
23-Feb-06
2.60
2.60
2.60
2.60
24-Feb-06
2.70
2.55
2.60
2.70
27-Feb-06
2.75
2.63
2.63
2.75
28-Feb-06
2.87
2.75
2.75
2.87

What trends do you see in this? Is it easier to view this way?

Raining Data February 2006 Stock Performance

Graphs are not just for PowerPoint (or Keynote) presentations. They are useful tools for examining numerical data - especially when there is lots of it - because it is easier for most of us humans to see and understand pictures than it is to glean subtle details from massive numbers of numbers. There are many opportunities within most database applications to make life easier for the end user by offering the option to display listed data in graphical form.

In the previous article, we dealt with mainly "surface" features of the Graph2 component. We examined how we set up a list variable to be used by the component, how to set various properties to achieve a certain "look" for a graph, etc. This time we will dig a little deeper into the workings of this component.

But first, some important Omnis Studio 4.1 setup information for Mac OS X users...

The Studio 4.1 Mac OS X Installation Shell Game

This past month I received communications from a number of Mac OS X Omnis Studio developers saying that they could not get the locked demo of my Graph2Lab product to open or that their copy of Omnis Studio would crash when attempting to place a Graph2 component on a window class in their own library. As far as I know, this is only a Mac OS X issue and it is easily solved.

If neither you nor your end users work on the Mac OS X platform, this issue will not concern you. It has nothing to do with the functionality of Graph2, but there are some issues with how certain things must be set up on Mac OS X that give many people problems with even following the examples in these articles. If your libraries that contain a Graph2 component will never be opened in Mac OS X, these problems will be meaningless to you. On the other hand, if you intend to deploy a library containing Graph2 on Mac OS X, you need to know this information...

I had originally included the details, specifics and cautions about setting up to use Graph2 on that platform in this section, but it got way too long. So I have made a separate HTML document with this information and placed it my Omnis Training web site under the Demonstration Libraries heading. Scroll down that page to the item labeled "Installing Studio 4.1 on Mac OS X".

Now let's move on to the good stuff!!

Graph2 Events

There are only two event types that are specific to the Graph2 component: evGraphClick and evPreLayout. Certainly there are other events to which a Graph2 window object will react, but these two apply only to the Graph2 field type and require special mention in this article.

The evGraphClick event is a special kind of click event used only for this component. In fact, a Graph2 window object does not generate a normal evClick event, so that event variable is not included in the event code selection list for the On... command for this field type. A Graph2 component does generate evMouseDown and other mouse events if the detection of mouse events is switched on for it (even though these events are not presented in the selection list either), but a common evClick event is not in the repertoire of a Graph2 object.

But the evGraphClick event does not simply mean that the user has clicked on the Graph2 object. Many clicks on a Graph2 field will not register as "graph clicks". The only time this event is generated is when the user clicks on a data element of the graph image on the Graph2 object. While the original Graph component allows us to also detect clicks on titles, axes, grid lines and even on the background of a graph field, Graph2 simplifies the programmer's role in the process of "drilling down" into the data behind the graph by only generating an event when a data element receives a click. Further good news is that this feature is always active as long as the Graph2 field is active. We do not have to switch it on as we do with the $selectobject property of the original Graph component.

In order to better define the specific graph click event, evGraphClick is accompanied by four event parameters: pSet, pSetname, pItem and pItemname. If we substitute "Group" for "Set" and "Series" for "Item", we are well on our way to understanding what these parameters tell us. pSet and pItem report the column and line number of the data element that received the graph click. pSetname and pItemname report the labels associated with that column and line. This allows us to determine the corresponding cell within the data list for the element receiving the graph click and to consistently label or describe any values we might report back to the user as a result of such a click.

Responding to a graph click is only an issue with Graph2 fields on window instances. Reports are not interactive, so a Graph2 object on a report cannot react to a click. Remote forms do not have Graph2 fields, but can only display pictures generated using the $snapshot() method of a non-visual Graph2 object in Remote Form Picture fields. But those Picture fields have been somewhat enhanced in Studio 4.1 in anticipation of another feature of Graph2, which we will touch on briefly later in this article.

The above is a description of how the evGraphClick event is supposed to work. At least, this is the theory behind the evGraphClick event. The results I have been able to achieve in practice so far have not fit the theory very well, but these issues have been reported to the proper people and may very well be fixed with the next release of Omnis Studio.

This is the end of the "bad news" portion of this article. Everything from here forward is good news!

An event that does work as described - and is quite useful as well - is the evPreLayout event. The evPreLayout event is somewhat unusual in Omnis Studio. It is triggered whenever the graph is being constructed. (Like the evImagePluginCreate and evXCompPluginCreate events of the HTML control field - but maybe you aren't familiar with those either...) This occurs both during the construction phase of the window instance and as a result of the execution of the $dispose() method of the Graph2 object.

The primary purpose of this event is to allow us to use the $add***layer() methods of the Graph2 component to build up a composite graph image by layers. But before we can use this event, we must also understand those methods (and vice versa)...

Graph2 Layer Methods

In the last article, we discussed the difference between the $redraw() and the $dispose() methods for the Graph2 component. We also examined how to use the $getcolors(), $setcolors() and $snapshot() methods.

But there just wasn't time nor room to also cover the $add***layer() group of methods in that article as well. As the names suggest, these methods are used to build additional layers onto the graph portion of a Graph2 object. Each new layer is placed behind the existing layers, just as each new series is placed behind its predecessors on an area chart.

We can only add layers of the same general type as the graph itself. So we cannot add a Polar layer to an XY chart. But we can add different kinds of XY layers to an XY chart - or different kinds of Polar layers to a Polar chart. (Only one kind of Pie chart has been exposed to us in the current Graph2 implementation. We cannot add Pie layers, so we have not been given such a method for Pie charts.)

For XY graphs, we can add the following types of layers:

Layer Type

Method

Line $addlinelayer(lList)
Bar $addbarlayer(lList)
Area $addarealayer(lList)
Scatter $addscatterlayer(lList[,iSymbol,iSymbolsize])
High Low Open Close $addhloclayer(lList)
Box Whisker $addboxwhiskerlayer(lList)
Candlestick $addcandlesticklayer(lList)
Trend $addtrendlayer(lList)

For Polar graphs, we can add the following types of layers:

Layer Type

Method

Line $addlinelayer(lList)
Area $addarealayer(lList)
Spline Line $addsplinelinelayer(lList[,iSymbol,iSymbolsize])
Spline Area $addsplinearealayer(lList[,iSymbol,iSymbolsize])

Notice that all of these methods require their own data list as a parameter. This list acts just like the one we use in the $dataname property for the Graph2 object itself, but these are completely different lists of values (and can be different variables). The list simply has to be structured correctly for the type of graph the layer will create. The first column will always be the Series name, for example.

The layout for each layer (portion of graph alloted to each XY group, how that area is divided for individual series bars, etc.) is determined by the shape (number of lines and columns) of the list supplied. Graph2 does not enforce any rules that the list for each layer of a given field must be the same shape. We must police this for ourselves - or let our creative juices flow...

Some mixtures of graph type work well together, while others do not. For example, we can use a Line layer to connect the closing value of a Candlestick or HLOC graph and the data points on the line will exactly overlay the point where the Close indicator meets the vertical line that indicates the High-Low range. But if we try to impose a Line layer over a Bar graph with multiple series, the data points of the line will be placed at the center of the area used by each group - and not in the center of the top of each bar. This is because the layers are independent of one another, so a Line layer knows nothing of the Bar graph in front of it or of the horizontal positions allocated to the series elements within a group. Adding layers is very different from telling a Bar graph to connect the tops of the risers for a series, as we can do with the original Graph component.

Also notice in the tables above that $addscatterlayer(), $addsplinelinelayer() and $addsplinearealayer() methods give us the option of specifying the symbol used for data points added by that layer, as well as the size of symbol to be used. (Curiously, the $addlinelayer() methods do not...) The symbol must be specified using one of the constants supplied with Graph2, but we can also use their numeric equivalents. Here are our options:

Constant Value
kG2symbolNone
0
kG2symbolSquare
1
kG2symbolDiamond
2
kG2symbolTriangle
3
kG2symbolRightTriangle
4
kG2symbolLeftTriangle
5
kG2symbolInvertedTriangle
6
kG2symbolCircle
7
kG2symbolCross
8
kG2symbolCross2
9

This is also the order in which these symbols are automatically used in a "normal" scatter plot. With an XY Scatter graph, we have no control over which symbols are used. But if we build our graph one Scatter layer at a time, we can choose any of the nine symbols provided for any layer - we just have to plot only one series per layer.

If we want to add layers in addition to the main graph built using the list variable named in $dataname, no problem! The first layer will simply be placed behind the original graph. But if we want to build our graph entirely with layers, we need at least the skeleton (structure) of a graph from the basic data list in order to have group titles appear on the graph. There are currently other limitations to such a graph. The main limitation is that we will see no legend if there is not at least one series in the base graph. The contents of each layer will add elements to the legend, but only if it exists in the first place.

So how do we go about adding a layer to a graph? We simply execute one or more $add***layer() methods during the evPreLayout event. We must supply an appropriate list variable as the first parameter of the method for each such execution. Suppose that we have a Graph2 field set up to present an XY Line graph and we want to build it using layers. Here is a simple example:

On evPreLayout
    Do dummyList.$copydefinition(dataList)
    Do dummyList.$add()
    Do dummyList.1.$assignrow(dataList.1)
    Calculate graphObjRef.$dataname as nam(dummyList)
    Calculate layerList as dummyList
    Do layerList.1.$assignrow(dataList.2)
    Do $cfield.$addlinelayer(layerList)

Here layerList is a local List variable. dataList is the main source of values for our graph. dummyList is a substitute for the dataList as the source for the main graph as a means of assuring that the group labels and the legend will be included in the resulting composite graph. This works fine for a line graph where one layer cannot obscure another layer. But for Bar and Area graphs, we may want to stack the layers. For this, we have to be a little more clever...

Building a Stacked Bar Graph

There are many situations in which we might want to add graph layers. One of these, which at first appeared to be a glaring omission within the Graph2 repertoire, is for the creation of a stacked graph. This is a graph that displays the values for all series for a group as an accumulated value, but then displays striations whose sizes are determined by the individual series values. Here is an example of a stacked XY Area graph:

Example of a Stacked Area Graph

This kind of chart allows us to easily see the overall value for each group and to see the relative contributions of each series entry to that total. It is more difficult to determine the absolute contribution of any series past the first one with this type of chart, but for that we would use a simple bar chart (which, in turn, makes it more difficult to determine the overall total for a group).

Simple Bar Graph

Let's suppose that we would like to stack the bars of this graph rather than show them as separate bars so that we can better determine the composite sales for each quarter as well as seeing the relative contributions of each region. That graph would look like this:

Stacked Bar Graph

Notice how each bar in the stacked graph takes up the entire area (width) for the group, while this area is split among the various series in the simple bar graph. As long as the shape of the list supplied for each layer is the same, the width and position of each bar in corresponding layers is also the same. Note also how the Graph2 object automatically adjusted the scale of its Y-axis to accomodate the composite bar height. That is because this aspect of the graph is determined once all the layers have been defined during the evPreLayout event.

The secret to generating a graph like the one above is that our code must accumulate values for each successive layer. That is, each layer must represent a single series, but each group value in that series must be the value for the current series plus the sum of the values of the series that have already been graphed. If we were to look at this graph in a 3D oblique view, this will become clearer:

3D View of Stacked Bar Graph Layers

The series 1 values were used as is. But the series 2 layer displays the sum of series 1 plus series 2 for each group. The series 3 layer adds the series 3 values to that total, and so on. All we need is some code to generate those cumulative values for each group. But what if we don't know in advance how many series or groups will be involved? We can determine this as we go. Let's examine the code I have built to perform this operation first, then we'll break it down:

On evPreLayout
    Do dummyList.$copydefinition(dataList)
    Do dummyList.$add()
    Do dummyList.1.$assignrow(dataList.1)
    Calculate graphObjRef.$dataname as nam(dummyList)
    Calculate layerList as dummyList

    Calculate linecount as dataList.$linecount
    For line from 2 to linecount step 1
        Calculate layerList.1.1 as dataList.[line].1
        Do layerList.$cols.$sendall(layerList.1.C[$ref.$ident].$assign(
                $ref+dataList.[line].[$ref.$ident]),$ref.$ident>1)
        Do $cfield.$addbarlayer(layerList)
    End For

First, we deal with our dummyList variable as detailed in the previous section. Then we set up the layerList variable as a copy of dummyList to begin the process. We also determine how many lines are contained within dataList and hold that value in the local variable linecount. Now we enter a For... loop using another local variable, line, as our counter. This value represents the dataList line (and graph layer) we are currently processing. Notice that we begin this loop on the second line of dataList. For each line we encounter, we copy the Series name into column one of layerList. We then execute the most complex line of code in this example to accumulate the values from the current line of dataList with the corresponding column values already in layerList. After our series values have been updated for the current layer, we add that layer to the graph. For further explanations of using the $sendall() method against the group of columns in a list or row variable (and to understand what all those "$ref" items and square brackets do), please read my book "List Variable Operations", available from http://www.omnistraining.com/reflibInfo.html

While this is a bit more work than having a straightforward stacked bar graph data type (as does the original Graph component), it isn't so much more work that we could call it "difficult". As with most things Omnis, it's just a matter of understanding how the basic tools work!

Coordinate Positions and Graph2 Elements

As mentioned earlier in this article, we cannot directly place a Graph2 object on a Remote Form. The Graph2 library does not contain such a component - only Window, Report and (non-visual) Object components. But we can create an object variable using the non-visual Graph2 Object component and make use of a new feature of a Remote Form Picture field added to Studio 4.1 primarily for use with Graph2-generated graph images.

The evClick and evDoubleClick events of a Remote Form Picture field have been given two new event parameters: pMouseX and pMouseY. These return the horizontal and vertical position of the mouse, measured from the upper left corner of the Picture field image (that is, excluding any borders) at the time the event took place. These measurements are in pixels. We can then apply these values as parameters of another method of the Graph2 Object variable (back on the server) that can determine which data element, if any, of the graph in the image was clicked upon. That method is the $findobject() method.

The $findobject() method of a Graph2 Object requires four parameters. The first two are the X and Y coordinates of a point within the Graph2 image. The third and fourth parameters are passed by reference, so they each receive a value as the result of executing the method. They must be variables of numeric type (they will be given Integer values) and they return the Set (Group) and Item (Series) numbers respectively of the data element in the graph that covers the point specified by the first two parameters. The return value of this method is a Boolean value that indicates whether the click was on a data element. If not, it returns kFalse - but it returns kTrue (and non-zero values for the variables named as third and fourth parameters) if the point coordinates given do, in fact, hit a data element. Think of the execution of this method as a notational "graph click".

There are optional fifth and sixth parameters for the $findobject() method as well. These are also passed by reference and must be Character variables. They return the Set (Group) and Item (Series) names (labels) associated with the data element at the point specified by the first two parameters. So this method is able to return up to five useful pieces of information based upon two supplied values.

Obviously, this method and the new event parameters for a Remote Form Picture field are meant for each other. But we can also use those event parameter values as a sort of image map coordinate for other purposes (although the HotPict component is probably a better choice for that).

Graph2 Text Tags

One final tidbit to pique your curiosity - and to solve some serious problems for those people who must use special fonts for non-Roman alphabets. Consider this the "dessert course", if you will. In the previous article on the Graph2 component, I stated that we have no control over the fonts or other text properties used in titles and other text-bearing elements of a Graph2 graph. It turns out that this is not entirely true. What is true is that there are no font-related properties of the Graph2 object. But what is also true is that there is a rich and robust system of tags that we can use within the values of text-bearing properties and other labeling content of a graph that allows us to do some pretty impressive things with text on Graph2 fields.

If the term "tag" is unfamiliar, let me briefly explain: A tag is a string (like the escape sequences we explored when we studied styled text a couple of years ago) that is used to specify attributes for a range of characters in a string value. Tags are separated from the text in which they are embedded by using a special sequence of characters to denote the beginning and end of the tag. For example, HTML tags begin with < and end with >.

Graph2 tags are similar to HTML tags. But Graph2 tags are signaled using the character combinations <* and *> to indicate the beginning and end of the tag. Within the tag, specific text properties recognized by Graph2 are given values in a propertyname-value pair. This specification takes the form:

propertyname=value

Unlike HTML tag parameters, no quotes are used to enclose the value string. In this article, we are only concerned with font tags, but there are other types as well. The basic form of a font tag is:

<*font=fontname*>

The word "font" is the name of the property and we assign it the name of a font file that is installed on our computer. There are some platform differences in which kinds of fonts can be used. We can use TrueType (.ttf), PostScript Type 1 (.pfa and .pfb) and Windows bitmap fonts (.fon) on the Windows platform. On Mac OS X we can use TrueType, OpenType, PostScript Type 1, Data Fork and even Font Suitcase fonts. We can also use Mac OS X Font Manager names if we prefer.

There are also some assumptions made as to where the fonts will be found. On Windows, Graph2 looks for .ttf files in the \Fonts directory of the main Windows installation. On Mac OS X, it looks in /Library/Fonts and /System/Library/Fonts. There are also ways to tell Graph2 to look in other directories, but this should be enough for right now.

So to specify that we want to use the Arial TrueType font, we would use:

<*font=arial.ttf*>

Once we have established that this is a font tag by using the font property, there are other font-related properties we can specify in the tag as well. For example, we can use the size property and specify the font size in points. So to specify 18 point Arial, we would use:

<*font=arial.ttf,size=18*>

The font tag must be included in the property value before the character string that it will affect. If our main title is "Sales by Region" and we want to display it in 18 point Arial, then the value we must give to the $maintitle property is:

"<*font=arial.ttf,size=18*>Sales by Region"

One other property that you may find useful is that we can override the default text color (assigned using the value in the third line of the color list for the Graph2 field) by specifying the color property value. This must be given an rgb color value as a hexadecimal string (but without the initial # character we would use in HTML). So to add a red text color to our main title, we would use:

"<*font=arial.ttf,size=18,color=FF0000*>Sales by Region"

Tags like these are usually hidden from the end user in most applications, but not so in Graph2. If we want to shield this programmer trivia from our end users, we must build an interface that allows the user to make font, size, color and text specifications without seeing the actual syntax of the finished property value. I have already done this work (and a great deal more!) for you in my Graph2Lab product. You can learn more about this on my web site at http://www.omnistraining.com/g2labInfo2.html. Here is a simple example of what we can now achieve in Graph2Lab:

Graph demonstrating use of fonts and transparency

Here we see a Graph2 Pie chart where the main title was given the Russell Square Oblique font at a size of 36 points. Each series label (each cell in the first column of the data list) was given the Optima Bold font and a size of 12 points. You might notice other interesting attributes of this graph as well.

If we add text tags to an existing graph, the $dispose() method must be executed to realize the effect. Depending on what other changes may have been made, this may also need to be followed by a redraw of some kind.

As with HTML, Graph2 tags can contain many more properties than the common ones we have reviewed here. We can also inject them anywhere within a Graph2 text value. So we can change font, size, color and other attributes on a character-by-character basis within Graph2 if we feel the need to do so. We have the power to create some truly ugly graphs!

For Further Study

I am compiling a more complete collection of this information as part of my Advanced Technique Monographs series for those who are interested in pursuing the advanced uses of the Graph2 component further. I have also added font control features for all labels and titles in my Graph2Lab product to give the Omnis Studio developer even more ideas on how to take advantage of these features. The trick is to make these tags invisible to the end user so that they do not break them.

Next Time

In the next issue of Omnis Tech News, we will return to our discussion of Object Instances - specifically, the subject of Data Objects. I hope you continue to find these articles useful.

 
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