It it the relation between the average of UP-DN signals Vs the the input phase error.
It can be plotted by running multiple transient simulations , each time you change the delay between the input clocks.
each transient simulation shall be left for a long enough time , in order to get an acurate average.
Also The PFD has another C/Cs which is the average of UP-DN signals Vs the the input frequency error.