The manipulated variable is plotted on the horizontal axis

True.

Manipulated variable or also called the controlled variables are variables in which you regulate. Manipulate or as said control. By this you want to certain the outcome of a certain experiment. Making it close to which you want it or desired it to be, possibly. Characteristics could be: 
1.       You are able to have govern or a certain variable is controllable. 
2.       The outcome or effect of a particular variable was what you were determining or were testing with high probability 
3.       You can either increase or decrease the level of which the variable could take effect on a dependent variable.