The hashrate chart we plot using the API : https://api.metapool.tech/address/YOUR_WALLET/history/hashrate?timebucket=15m. We not sure the website API used (the plot using same 15m), but we get different result in charting data. Using the current hashrate data, we create average plot data using 3H moving average. NOTE: We have asked to metapool admin and they give formula related with different result in hashrate chart. We will update this in the next release.