You may chose to plot your data as a function of frequency, parameter values or optimization iterations. The values chosen appear on the bottom axis of your plot.
NOTE: If no parameterization or optimization analysis has been run on this project, i.e. only a frequency sweep analysis, then only Frequency is available to plot over. In this case, the only choice enabled in the Plot Over menu is Frequency.
If you choose to plot your data as a function of frequency, then select Graph - Plot Over - Frequency from the response viewer main menu.
If you choose to plot your data as a function of the parameter values, then select Graph - Plot Over - Parameter from the response viewer main menu.
If you choose to plot your data as a function of the optimization iterations, then select Graph - Plot Over - Optimization Iteration from the response viewer main menu.