AFAIK, git doesn't have a feature for word counting, let alone drawing graphs of word counts. However, you can combine git and other tools to do that.
For example, this will output word counts for each commit that changed file.txt
file:
git rev-list HEAD -- file.txt |
while read c; do git show "$c:file.txt" | wc -w; done
And this will output the same along with commit timestamps:
git rev-list --timestamp HEAD -- file.txt |
while read t c; do echo -n "$t "; git show "$c:file.txt" | wc -w; done
You can feed this output to gnuplot, or paste it into openofice.org Calc to plot a graph.