Which is the independent variable in the graph of the U.S. population 1800-1900?

In the graph of the U.S. population from 1800 to 1900, the independent variable is typically time, represented by the years along the horizontal axis. This variable is manipulated to observe changes in the dependent variable, which, in this case, is the U.S. population size shown on the vertical axis. By analyzing the independent variable over this time period, we can see how the population has grown.