Support bubble charts

[eskroch]

Moved from Google Code issue #534.

Add support for bubble charts. These are like scatter plots, but the size of the plot symbols represent a third data value. It could probably be implemented as a subclass of CPTScatterPlot.

Aug 10, 2013 comment #1 mike.lischke

Note that there are different types of bubble charts. One is similar like a tree chart. The bubbles are not drawn along a horizontal axis but grouped in a given outer area (which can again be a circle like in this example or a rectangle).