lisa.stats: Fix plot when there is one group only
lisa.stats: Fix plot when there is one group only

Instead of removing all the group's description, keep it all (even if it
is redundant with all the other groups, since there is only 1).

credp/lisa,credp/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa