Comparing the clusters size distributions

GROMACS version: 2018
GROMACS modification: No

Dear all,
I want to see the effects of a variable on clusters size distribution on a system. The gmx clustsize gives the cluster size distributions in *.xpm file which are matrix, in fact. I can visualize each of the *.xpm file using the xpm2ps, however I can not really compare the *.xpm files. Any solution or idea for doing that would be highly appreciated.