Fix Y-axis step calculation in graphs

Y-axis calculations previously dictated a set number of ticks, which sometimes led to cells having too much whitespace at the top, or overrunning the top of the graph. This commit allows the number of ticks to adjust to fit the maximum height of the graph.

Each graph cell calculated its maximum height and Y-axis ticks; this commit moves that calculation to the graph background to avoid rerunning those calculations for each cell.
Merge pull request #53 from jmdodd/issue/32-graph-view-y-axis

Fix Y-axis step calculation in graphs