As of commit 37df824b5316cd4cd1ce4734a5a4f3f9d967fb9c we have two graphs:
The old graph based on difficulty (turns out it's not as inaccurate as I thought)
A new graph called Blue Hashrate. This graph polls EstimateNetworkHashesPerSecond for the virtual block. I'd consider it a bit less accurate that the old graph, since it doesn't take red blocks into account
Currently it's using an outdated algorithm based on Bitcoin difficulty