fix plot off-by-one problem

The last data point of the plot was missing, because it was plotted
beyond the xlim(). And drawstyle="steps-post" only draws a short
vertical line for the last point; add a final point to fix this.

And center the bars on the x-axis on integer samples.
Merge pull request #217 from martinxyz/master

separation_plot.py (Chapter2): fix plot off-by-one problem