fix unpleasant binning caused by numeric error

A plain division of value offset by step size could assign a value to
'wrong' bin. E.G., floor(0.15 / 0.05) = floor(2.9999...6) = 2.

We now round the quotient to certain precision digits so as to
tolerate the numeric error and make the binning more in favor of human
interpretation.
Merge branch 'master' into summary2