This was requested on the mailing list and it makes sense to
have it. Of course, not all charts make sense: e.g. a plot dive-#
vs. count is a bit redundant...
Sadly, this can't use the generic IntRangeBinner, because dive-#s
start at 1, not 0.
Suggested-by: Christof Arnosti <charno@charno.ch>
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
83848fb2a8