Compound charts: Expose data series count property (patch attached)

I needed to add a line chart overlay to an existing bar chart.
As http://code.google.com/apis/chart/image/docs/gallery/compound_charts.html states, it's only a matter of explicitly setting the data series count in the chd property and setting the line markers via chm.

Suppose you have 3 series of data and another one to overlay. You just populate the data array with the 4 series as usual, being the overlay series the last one to be populated.
When done, just set #data_series explicitly to 3 and adjust markers like this:

  //Populate your 4 data series

  $chartg['#data_series'] = 3;
  $chartg['#shape_markers'] = array(array('D','000000',3,0,3));

  //Render your chart as usual

The above #shape_markers array will draw a 2px wide black line (D) using the third data series.
It's working for me on both dev and prod sites.

I was aware of #1250720: Line Markers and Circle Markers integration for making Composite Charts through Views, but it was not working for me. Since I want to modify as little as possible the prod site and I'm afraid of using -dev code in a prod site I did it this way. I just changed my patch original property name to #data_series to match -dev when it gets stable in the future.