showstack -> show_stack
Merge pull request #554 from ndawe/master

[MRG] showstack -> show_stack
ratioplot template
add fill_value argument to Hist.divide
RatioPlot: set tick lengths
RatioPlot: draw horizontal lines in ratio pad
_limits_helper: don't raise ValueError and instead return reasonable values if min=max
templates: center ratio plot y-axis title