New Chart Types in Office 2016

[perkes]

Office 2016 introduced several new types of charts that are currently not supported by python-pptx: Waterfall, Histogram, Pareto, Box & Whisker, Treemap and Sunburst. The XMLSchema for these chart types seems to be different to those charts previously available. The most obvious difference is the names of the tags cx:chart instead of <c:chart>, actually all tags are cx instead of c. I tried adding a new class to xmlwriter.py and a new enum to enum.chart but that proved to be insufficient. It seems many parts of the code assume the tags to be named <c:.*>.

It would be great if we could collaborate to support all (or at least some) of these new chart types.