work on labelling fix for PolarAreaChart

I hesitate to submit this pull request because of my general ignorance with polar coordinates, but this fixed the labelling problem that I mentioned in [issue #1](https://github.com/biovisualize/micropolar/issues/1).

The chart now appears http://bl.ocks.org/timelyportfolio/6875359 versus http://bl.ocks.org/timelyportfolio/6872354.