Support for multi-dimensional array of plotData for CPTBarplot

[VikramParimi]

Why is that we need to create multiple instances of the CPTBarplot to have multiple bars at the same location?

It would be great to have support for the multidimensional array as a data source to the barPlot.

Along with this, there must be a provision to toggle between stacked and grouped flags to maintain the grouping of the bar plots and works as below:

stacked: plots will be stacked upon each other at each bar location
grouped: plots will be rendered adjacent to each other center aligned to the bar location.

[eskroch]

This would require a major architectural change to the way plots work. Stacked plots have been on the todo list for a long time (see issue #11). I'm open to ideas on how to implement this kind of change without breaking a lot of legacy code.

[VikramParimi]

We can add a new plot like CPTMultiBarPLot which internally creates CPTBarPLot instances based on the count and they can be aligned as grouped or stacked(based on the developer requirements). This helps in keeping the legacy code intact.