full_range_monthly.gnuplot: fix sampling when drawing boxes

Instead of working around the problem by doubling the sample rate, fix
the actual problem and prevent different samples to refer to the same
box, resulting in holes in the background.

The solution is to have samples fall in the middle of months instead of
their boundaries, this compensates for the fact that different months
having a slightly different number of days.

Now two different samples are always in two different months.