Here is a very basic indicator combining the volumes of the 9 biggest exchanges trading BTC/USD or BTC/USDT. These 9 exchanges were chosen based on the report by Bitwise Invest stating that 95% of the volume on CoinMarketCap is fake. On these 9 exchanges, however, volume data appears to be reliable. Please note BitFlyer was not included because it does not trade in USD. Please note also that data on all 9 exchanges is only available from June 2018.
Anyone is welcome to modify this and make it more elegant, this was just a quick implementation.