Reconsider using relative box and bar widths.

Original [issue 133](https://code.google.com/p/plotinum/issues/detail?id=133) created by eaburns on 2013-07-17T13:22:42.000Z:

Currently plotter.BoxPlot and plotter.BarChart (maybe others) use absolute values for their widths. It can be hard to predict a good width, and currently one either needs to do very roundabout things (see this discussion: https://groups.google.com/d/topic/plotinum-discuss/WD02Ty30G2c/discussion) or to have a good guess. Perhaps using a relative width would be better. Can we make a nice API that supports both?