Motivation: ease plotting of several snapshots coming from different
triplets of shapefiles, on the same figure. We may want to do this if
we have anticyclones and cyclones in separated triplets of shapefiles.

In order to extract the code for this function from the main part of
the script, we have to modify the behaviour of the script a little:
when the file `ishape_last.txt` could not be read or there is a single
value in it then args.k, if not None, must be equal to k1, instead of
being ignored.