Every broker has a different market data vendor. The prices and the spread level may vary between the platforms. Our feeds are obtained from Bloomberg, who are arguably the most renowned market & financial data vendor in the whole world.
As @phildawson pointed out - Popular publicly available sources such as Google or Yahoo Finance do not include spread as they don’t quote BID/ASK but rather LAST TRADE - the last price at which a trade was executed, not the current best bid & offer. Also, they’re usually delayed by 15 minutes.
In addition, please have in mind that when it comes to the Invest and ISA accounts, the graph is strictly indicative, however, rest assured that orders are being executed at the best available market price.