Log scale doesn't work properly with multiBarChart

[SrslyJosh]

Plunker here: http://plnkr.co/ncczw1waN5jzqBI63H7Y

I found a hack that will get this to render properly (by scaling Y values with a function), but that of course results in incorrect values being displayed in the tooltips when you hover over a bar.

I believe this should work, but I can't find or figure out the correct way to do it.