Yet another sort order for memory footprint

Given a vector of (x, y) representing x accesses to each of y unique bytes,
the "X bytes cover Y% of memory accesses" histogram originally sorted in
decreasing order of y.  Commit b783b25a2a1439a1c2e11e46232a19688466c323
changed the code to sort by decreasing order of x*y.  The current commit
once again changes the sort order to be in decreasing order of x.  See the
discussion in issue #9 for details.