That graph is for the number of transaction, not transaction volume. Bitcoin has a max amount of transactions per block and blocks have been more or less satura

That graph is for the number of transaction, not transaction volume. Bitcoin has a max amount of transactions per block and blocks have been more or less saturated since a while so the number of transactions can't actually increase. It certainly doesn't look like it's decreasing. A lot of the transactions has been moving to second layer solutions as well, with on-chain becoming a sort of settlement layer.