Chirp is the instantaneous change in frequency, derivative of the phase w.r.t time. To measure the chirp in the plot you attached, you need to choose one of the polarizations to inspect. Click the Power X or Power Y tab. Then you can enable the Analysis checkbox on the right and choose to display the Phase pr Chirp.
The chirp and phase are not well defined when the power is close to 0 therefor it makes more sense to measure the chirp of individual pulses.
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