diffstat: scale adds/removes proportionally to graph width
The previous method of scaling had a tendency to include graph lines that
went past the output width when the file with the most changes had a very
large number of changes.
M a
M a
M a
M a
M a
M a
M a
M a
M a
M a
M a
M a
M a
M a
M a
M a
M a
M a
M a
M a