Git's pull output has been explained here fairly well. In spite of this I'm still unsure exactly what the text graph relates to.
For example:
git diff --stat master HEAD^
Outputs (truncated):
Site/index.php | 118 ++--
While the number of lines modified is clearly displayed as 118, the text graph is a little harder to interpret.
Could this relate to the ratio of added and removed lines?
Yes it's the ratio of added and removed lines.
See also:
man diffstat