The x-axis of the graph is the time while y-axis of the given plot is the money inside Lucy's purse in dollars. we can see that from 6 am to 11 am, Lucy's money is equal to about $60 while around 11 am it rose up to around $125 and then decreased to $100. This means that in the given range of time, only one transaction per time range was done by Lucy. One scenario here was that before lunch, say 11 am, her money was $60 and found out that she has to buy something after work, in which her money was not enough. She went to withdraw some cash that's why she had $125 in total and has to spend a portion of it at lunch with the remaining cash of $100.
