In the standing wave experiment, we plan to measure the speed of the mechanical wave on the string. So we measure the frequency for each mode f , and...

In the standing wave experiment, we plan to measure the speed of the mechanical wave on the string. So we measure the frequency for each mode f , and the wave length of each mode l. The equation is f=v/l. To got speed v. we use linear fitting on the graph. The value on y axis is f. What is the value on x axis?