Fix frange rounding errors when generating ylabels.
Fix frange rounding errors when generating ylabels.

For example, when yMin=0, yMax=7 and yStep=1.2, the calculated
yTop=7.199999999999999 (`1.2 * Ceil(7/1.2)`), which results in frange
returning (0.0 .. 6.0) instead of the expected (0.0 .. 7.2).

errx/carbonapi,errx/carbonapi,dgryski/carbonapi