fix row chart labels so they are vertically aligned *always*

This fixes it so rowchart labels are *always* vertically aligned, unless you've added signifigant `offsetYLabel`.  I'm running a lot of row charts of various heights, with varying numbers of rows, and these tweaks seem to do the trick.  

`_labelOffsetYdefault` is simply to track whether the user has overridden the sane default through the API, and if so, we don't muck with it and leave it to whatever they set it to.