Fix 0.0 value on axis to be 0.0.

I recently saw cases where the %g format was cauing a value that
should be 0.0 on the axis to appear as something insane like 5.58e-17.

If the value is within a thousandth of a sub-step, (a sub-step is the
space between two minor ticks on the axis), of 0.0 then force it to be
truly 0.0.