histo2d: Option -r to set a reference range (1)

Read reference range from a file selected by option '-r'.
It must contain two lines with the minima for each column
on the first line and the maxima on the second line.
histo2d: Option -r to set a reference range (2)

Set binning range such that the reference interval is
divided into the wished number of bins and then more
bins are added or removed to cover the data interval.
histo2d: Option -r to set a reference range (3)

Allow for reference minimum smaller than data minimum.