You’re not the only person to ask about this so I’ve moved this to it’s own topic to make the answer to this question easier to find 
This one’s a bit confusing - the percentage for the stock price graph is calculated based on the price when the market closed on the previous day, whereas the first point on the graph is today’s opening price. Since the price will change in after-hours trading, there’s often a difference between the two prices.
We’re planning to improve this in the future!