Add a class for plotting data and a method to plot data as a stacked bar chart

A new class will be provided which holds plotting functionality. A possible new method to plot data as a stacked bar chart takes a well-formed data-frame as input and generates a stacked bar chart based on that data-frame. The data accepted could be aggregated counts based on answers given for each sub-question involved.

Assumption:

Data-frame could look something like this as for question G4002 (which contains 12 sub-questions with 4 possible predefined-answers):

(Index) SQ001 SQ002 …
Never     12    34  …
Sometimes  3    14  …
…

Or the transposed data-frame:

(Index) Never Sometimes …
SQ001     12      3     …
SQ002     34     14     …
…

The transposed data-frame seems to be more promising.