Uploaded an example code to calculate powerspectrum of time-sliced XIS data to this gist entry. Enjoy.

https://gist.github.com/yuasatakayuki/7b622096bcb8443c413b

This script generates PDF files something like the following. The x axis labels are only displayed for the bottom panel. In QDP, you can turn on/off the axis labels by doing like:

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lab nx off
lab ny off