Greyscale Bitcoin Investment Trust (GBTC) is a U.S. exchange based trust that represents a fractional amount of Bitcoin. For details, see: https://grayscale.co/bitcoin-trust/
GBTC has many downsides to just outright owning Bitcoin, but it has convenience and also can be placed in retirement accounts. GBTC is a derivative investment- so charting it has risks not inherent to the underlying. Because GBTC trades at a premium (that shifts constantly) to the price of bitcoin, it is a great indicator of the opinions of stock market investors.