Obtaining power over time with GNU radio

I recorded a signal to a file using a SDR device and GNUradio. The file is complex binary file.
I would like to obtain from this file the power of the signal (in dBm) over time.
How can I save this data using GNUradio, or MATLAB? I would like a log containing 2 columns, one of them is time from the file beginning, the other is the power at a certain frequency. Another possibility is to display the power at a certain frequency (in dBm) as a function of time on a graph.
Appreciate the help.