added .fixedBarHeight([height]) to rowChart - useful for TopN charts

This was a UX request from a designer, and I found it useful.  Sometimes the rowChart autoscaling looks a little fat and chunky, say for example 2 rows when it normally shows 10 rows.

We use TopN charts, and graphically it is appealing if the bar height always stays the same.  So if we do Top 10, we always show bars that are height / 10 (plus gap).  Then we don't get fat bars.
The documentation for this parameter shows how to calculate bar height with a gap for nicely formatted viewing.