Track Commits. Not Time.
No, man. It's certainly useful to read a developer's commit history. But at they end of the day they're just as silly and misleading as any other metric.
The only metric that matters is business value generated. A really valuable developer might be able to make important strategic contributions to a project through just a handful of commits. While a particularly weak (or negative-performing) committer might be making zillions of commits, but actually doing irreparable damage to the project before anyone notices.
Those activity charts might make good eye candy, but if you don't know your developers or understand their working styles, they just won't tell you that much.