Handle yMin(Left)/yMax(Left).

Graphite normally uses yMin and yMax but these cease to have any effect if
you're using dual-axis mode. Add code to scan through the graph fields and
if any field sets "second_y_axis", switch to using yMinLeft and yMaxLeft
instead where appropriate.
Merge pull request #42 from bodgit/dual_axis

Handle yMin(Left)/yMax(Left).