The given graph is about the variation of price in certain days. The ordered pair (1, 91.50) means that on day 1 the price per share was $91.50.
On day 3 the price is 92.75.
On day 4 the price is 90.50.
To find the variation, we just have to subtract
Hence, the price decreased by $2.25.
We repeat the process to find the increase between day 4 and day 5.
Hence, the price increased by $0.25.