While not quite as good as having it implemented natively, I thought this was a pretty nifty idea and went ahead and added it to [my userscript](https://stackapps.com/questions/2575/stack-exchange-reputation-graph-total-reputation-overlay). In addition to the overlay in the top left corner, users of the script will also have an option to toggle the "Network Total" reputation series on and off. For convenience, the toggle setting is persisted in local storage.

Example of the script in action:

![network reputation graph with userscript's "Network Total" series enabled][1]


  [1]: https://i.sstatic.net/Gm4CU.png