Fix issue with Bubble radii

The radius of a point was relative difference between the point and the
smallest point, rather than the absolute radius of the point.

We now use the absolute radius of the point, as well as adding tests and
updating the final diet.
Make bubble width depend on chart size

For larger charts (with smaller ranges), we can afford to have larger
bubbles. The reverse is true for small charts with large ranges.
Merge pull request #55 from tpkelly/master

Fix bubble chart radii bugs