During the 1920s, people would buy stock on margin, which meant that they

Buying shares with margin implies borrowing money from a stockbroker to acquire a stock. You can imagine it as a loan made from your brokerage company. Purchases with margin allow you to buy more securities than your budget would normally allow you. By operating with margin you get great market exposure, depositing only a percentage of the capital you will use. The margin corresponds to the deposit you need to open a position, defined by the margin rate that your brokerage or broker company gives you.

well considering i can't see the graph, it's probably prices