The controlled variable in the experiment that results in this graph is the number of times the brand of gum is chewed. There are several brands of gum that are chewed, and that makes this factor not to be the controlled variable. The brands are Big Red, All Day Chew, Hubba Bubba, Stride, and Trident. .
Also, the size of the bubble of gums that were chewed is different also, with 3cm for Big Red, 8cm for All Day Chew, 10cm for Hubba Bubba, 6cm for Stride, and 4cm for Trident. This makes the size of the bubble not to be a controlled variable.
A controlled variable is another thing that you also need to understand. It is a kind of constant that is not necessarily shown in the outcome of an experiment but is a key factor in determining the outcome of an experiment. There can be more than one control variable in an experiment.