Generally, while drawing a graph, the independent variable is placed on the x-axis and the dependent variable in placed on the y-axis. In economics, the opposite takes place. In demand and supply curves, we see that price is placed on the y-axis and quantities are placed on the x-axis. Thus, despite being the independent variable, price is placed on the y-axis.

Now, why does this happen in economics? The answer to this question takes us to the history of demand and supply curves.

The demand curve was firstly introduced by Alfred Marshall. He popularized this graphical analysis technique in his famous book "Principles of Economics". While writing the book, he used to note down how many units were brought in, for different items, at a given price (from local markets of London). He thought of quantity as the independent variable, with prices adjusting to clear the market. Thus, he drew the demand and supply curves with price on y-axis.

Since then economists followed the convention and it became common practice to put price on the y-axis.