Allow graphs to have non-zero bottom of Y axis

[EvanKrall]

Some stats that I look at have large, fairly stable values. On these stats, a deviation of only a few percent is meaningful.

Unfortunately, the lowest value on Y axes is always zero (or the minimum value of my stats, if stats actually go negative). I'd love to have the option to disable this behavior, and allow my data's minimum value to sit at the bottom of my graph.