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APL for the Practical Man

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Towards a Chart Wizard

September 20, 2023

What might a good, embeddable, re-usable chart wizard, wrapping the SharpPlot library, look like? This wizard should be easy to add to an application, and could be used in the Dyalog APL session as well. Before designing a GUI for a chart wizard, however, we must design an appropriate, lower-level wrapper for the library.

SharpPlot is powerful, comprehensive, and well designed. Despite this fact, we still need a thin layer over the top of it to insulate us from the details and idiosyncrasies of .NET, to have a clearer separation of source data and chart properties, and to provide a more declarative language for defining charts. For example, there are many "set" methods like SetMargins and SetPenWidth that are conceptually just properties. We should be able to specify them as such. There are also draw methods for each chart type, and we should be able to specify a chart type property rather than call a method. Finally, there is often overlap of the the primary data of a chart with the argument to the draw method and some additional properties. For example, the DrawBarChart method requires one or more vectors of bar heights. The corresponding categories, however, are specified via the SetXLabels method. The vast majority of the time, the categories and the corresponding values will be provided to us as two or more columns in a table, and we should be able to specify the data as such, and have our chart language take care of the rest.

For the sake of simplicity, we assume the data is provided in a namespace enclosing a vector of names, and a corresponding vector (of vectors) of values. The particular form of this data is not important. What is important is that it is essentially a table, with named and ordered columns. Using the data from the first example in the SharpPlot tutorial on bar charts we can construct the data space d:

      v←⊂'Project'('A2' '18' 'Q5' 'T6' 'T8' '32')
      v,←⊂'Last Year'(6 18 27 31 40 43)
      v,←⊂'This Year'(17 33 42 54 71 78)
      d←⎕NS''
      d.(Names Values)←↓⍉⊃v

Defining a chart is then an exercise in specifying properties:

      c←New 0
      c.ChartType←'BarChart'
      c.Heading←'Comparative Spend 2004-5'
      c.BarChartStyle←'TicksBetween,ValueTags,ForceZero'
      c.XAxisStyle←'XAxisStyles.MiddleLabels,XAxisStyles.GridLines'
      c.ValueTagStyle←'Vertical,Inside,SectorValues,RecolorOutside'
      c.ValueFont←'"Arial" 10 FontStyle.Bold Color.LightYellow'
      c.ValueTagFormat←'###0 Units'
      c.Gap←0
      c.GroupGap←0.5
      c.Colors←'Color.Green Color.Maroon'
      c.XLabelFormat←'Proj-XX'
      c

Where New just creates a namespace with a few default properties:

New←{
     p←⎕NS''
     p.Size←360 240
     p.Multi←0
     p.Scale←'Shadowed'
     p.Select←''
     p
 }

Which produces:

Comparative Spend 2004-5 Created by Causeway SVG engine - SharpPlot v3.71.0 Paint the paper ===== Border ===== Y-axis labels 0 10 20 30 40 50 60 70 80 Heading, subheading and footnotes ===== Heading Comparative Spend 2004-5 Region ===== for X-axis labels Proj-A2 Proj-18 Proj-Q5 Proj-T6 Proj-T8 Proj-32 X-Axis Ticks ===== X-Axis Grid X-Axis tickmarks Y-Axis Ticks ===== Y-Axis tickmarks Start of Barchart =========== Axes ===== Data value labels ... Flip colour 6 Units 18 Units Unflip colour 27 Units 31 Units 40 Units 43 Units Data value labels ... Flip colour 17 Units Unflip colour 33 Units 42 Units 54 Units 71 Units 78 Units Key ===== Block key Block key Last Year This Year Reset to original origin

Let's note a few things about this chart definition, comparing it to the C# definition.

First, all property values are simple numeric or text vectors or scalars.

Second, SharpPlot methods like SetValueFont and SetColors are called by setting the properties ValueFont and SetColors.

Third, a text vector will be evaluated if the property it is defining is a cover for an underlying SharpPlot method, as is the case for for ValueFont.

Fourth, in evaluated property values, double quotes are replaced by single quotes before evaluation.

Fifth, the various style properties that are normally specified as the sum of one or more enums, like BarChartStyle and YAxisStyle, are specified as comma or space delimited strings. In addition, the dot-qualifying name may be elided if it refers to the name as the property itself.

Finally, the ChartType property determines which draw method is executed.

Now, given the chart definition space c and the data space d, we can create the SVG v with:

      v←c BuildChart d

Note that the ChartType property defines how the data is interpreted, and how specific properties will be automatically set. Any property set by default for the chart type by may also be set by the chart definition. For example, for a bar chart, the XLabels (cover for SetXLabels method) property is automatically set using the values from the first column of the data. Nothing prevents the user from specifying the XLabels property explicitly in the chart definition, overiding the default behavior.

The Input Data Table and the Chart Type

In the example above, the input data table consisted of 3 columns, Project, Last Year, and This Year. Each ChartType processes the input data table by default in a certain way. For a bar chart, the first column is taken to be the category, and all subsequent columns are taken to be values passed to the DrawBarChart method. Thus, in the case above we get a multi-bar chart, with two bars per category. Most of the time it is easy enough to simply pass the proper table with the proper columns in the proper order to the BuildChart function, from the calling application. Sometimes, however, we have certain columns we want to ignore, or that will be used to split or group data inside SharpPlot. Alternatively the columns may not be in correct order. To facilitate this, we provide a Select method that specifies a list of column names. This is used to select, re-order, and by definition, omit, columns from the input data table for the default processing of the chart type.

Multi Charts

One nice feature of SharpPlot is the ability, for certain chart types, to do multiple mini charts in one go. We add a Boolean property Multi to cover this. If set to 1, then each series is drawn on it's own chart.

A Fly in the Ointment

The chart definition outlined above is just a set of name/value pairs. They can be executed in any order, as they just set the state of the SharpPlot chart object before the appropriate draw method is executed. But there are times when we may want to draw additional things on charts, additional graphs, or lines, perhaps in different colors, and it is important that additional properties are set after the draw method is executed, and then we need to specify the next draw method. Order becomes an issue here. One solution is for BuildChart to accept a list of name/value pairs rather than a namespace. As a chart definition needs to be stored to disk somehow, and a namespace is no good for that, we will need this feature anyway. Now we will have order. The namespace is then just a convenience for writing charts in code, rather than defining the chart in an array of name/value pairs. The result of the wizard should be this array.

Next Steps

Now that we have a tidy way of defining charts in code or as an array, we can think about a GUI to make it really easy. This is perhaps a good opportunity to look at the HTMLRenderer for a cross-platform solution.