Minor plot.violin bug and documentation fixes.
Minor plot.violin bug and documentation fixes.

-Using a pd.Series for `positions` with a 0 position would fail
 because using `0 in positions` would check if 0 was in the index,
 not the positions.
-x ticks would not be properly labeled under some circumstances
 because the plot was not properly drawn initially.

taborlab/FlowCal,taborlab/FlowCal