Estimate historical Bitcoin hashrate over a fixed window
A historical hashrate chart is reproducible when each point records its end height, sample window and unit. Bitcoin Core can calculate such points directly.
Use a fixed height on mainnet
For a node that has reached height 850000, compare two observation windows ending there. The return value of each call is estimated hashes per second for the network.
bitcoin-cli getnetworkhashps 120 850000
bitcoin-cli getnetworkhashps 1008 850000
Choose the scale for the chart
Divide by 10 to the power of 18 to display EH/s. Short windows react more strongly to irregular spacing between blocks. A longer sample smooths the chart but can hide changes within its interval. The estimate covers aggregate network work. It cannot identify individual mining devices or measure the hashrate of the host running the RPC.