When we draw a graph, we represent the Dependent Variable on the Y-axis and the Independent Variable on the X-axis. The graph is then named in the format "Y-variable vs X-variable". In this case Y-variable = mpg and X-variable = speed of car.
Independent Variable simply means that we can freely change that variable and observe how the Dependent Variable changes. So, here the speed of the car is being changed on a straight flat road and its effect on mpg is being studied.
