How can I plot a Bode diagram from a chirp signal without using tfest?

Hi,

I am identifying a Piezo actuator. For that purpose, I want to identify the transfer function of the system. I am using an (exponential) chirp signal as input to generate my output data. Now, i would like to plot the Bode diagram from the actual data before estimating a fit of the transfer function using tfest. The key problem is that I don't know how to exactly determine the phase shift if the frequencies are changing all the time.

Is there an easy way to solve this problem?

In the end, I would like to generate a diagram that both contains the actual Bode diagram and the fitted function using tfest.

Thanks!