the empty quarter

In a previous post we described how to set the series palette dynamically for a pie chart. A slightly different approach is required for area charts.

Let’s recap on the problem at hand: in what is a fairly common scenario, one chooses the palette colours to match with particular category values. For example, in the chart below “High” is represented in green, “Mid” in orange and “Low” in red.

However, if for a particular dataset of the of category values is not present, the palette colours are still used in the same order, meaning the “Mid” area is now shown as red and “High” as orange. This is confusing for the end user.

Once again we can solve this issue with some scripting of the chart. In this approach, we will override the beforeDrawSeries method to check for category value and set the fill colour based on that:


function beforeDrawSeries( series, isr, icsc )
{
  // get class of the renderer
  var type = isr.getClass().toString();
  // check for area chart renderer
  if( type == "class org.eclipse.birt.chart.render.Area" )
  {
    // get global counter
    // 1* forces conversion to int
    var i = 1 * icsc.getExternalContext().getScriptable().getPersistentGlobalVariable( "seriesCounter" );
    // get category value
    var val = isr.getSeriesRenderingHints().getDataPoints()[0].getSeriesDisplayValue();
    // set fill colour based on category value
    if( val == "1-Low" )
    {
      isr.getSeriesDefinition().getSeriesPalette().getEntries()[i].set( 242, 88, 106, 128 );
    }
    else if( val == "2-Mid" )
    {
      isr.getSeriesDefinition().getSeriesPalette().getEntries()[i].set( 232, 172, 57, 128 );
    }
    else if( val == "3-High" )
    {
      isr.getSeriesDefinition().getSeriesPalette().getEntries()[i].set( 128, 255, 128, 128 );
    }
    // increment global counter
    i = i + 1;
    // "" forces conversion to string
    icsc.getExternalContext().getScriptable().setPersistentGlobalVariable( "seriesCounter", "" + i );
  }
}


You may have noticed that a global variable seriesCounter is used to hold the reference to the current series. This is because the beforeDrawSeries method is called several times, once for each category value present in the data. Another bit of code is therefore required to define the seriesCounter variable in the initialize method of the report:


reportContext.setPersistentGlobalVariable( "seriesCounter", "0" );


One more point to note is that reportContext.setPersistentGlobalVariable only accepts string values, hence the “hack” to convert between a number and a string every time the value is retrieved or stored.