better tick spacing control for Y axis group

[landret]

Hi, in addition to setYAxisTickMarkSpacingHint() which defines a tick spacing in pixel, it would be great to have a tick spacing for each axis group in the same unit as the axis group min and max.

The idea is that the tick spacing in pixel would define the minimum possible tick spacing to display and would take precedence over the tick spacing in the axis unit, in case the chart window height is too small.