hplot: update diff-plot reference
hplot: update diff-plot reference

Using H1D histos with variable bin size uncovered a bug in how bins'
edges were calculated.
The new way (using variable bin sizes) is actually correct.

For a H1D with 20 bins, from -4 to +4:
 x=-3.6 was put into bin 0
 x=-3.2 was put into bin 1

but bin-0 is [-4.0, -3.6)
and bin-1 is [-3.6, -3.2)

with the new way of computing bins:
 x=-3.6 => bin 1
 x=-3.2 => bin 2

go-hep/hep,go-hep/hep