Round up fractional ideal points to avoid fractional point tooltips

This PR fixes #104 and includes some small code refactoring.

I experimented with a number of options for fixing the fractional point tooltips. I tried displaying tooltips like "2.333" as "2+" or "~3", but the tooltips looked funny when the ideal burnline was clearly crossing the line at a different value.

In the end, the approach I liked best was to snap the ideal burnline itself to whole point values, so there is no such thing as "completing" a fractional number of points. However, this means the ideal burnline is no longer a straight line, which may confuse some users.