auto adjust scale of y2 axis on the top graph

[crashb]

Resolves: #2 The distance between tics on the y2 axis was previously hard-coded to 10. After a certain point, these values became very hard to read. I have changed it so that there will always be ~6 evenly-spaced tics.

(Note: I have not yet had a chance to gather much data in mastostats.csv. Ideally, this should be tested when there are large values on the y2 axis.)