How to change the amount of data stored in pnp4nagios

I wrote a Nagios check plugin using historical data from sar that you might be interested in. Even if it's not useful to you out of the box, you can easily use it as a starting point for more sophisticated checks.

The only possibility I see is writing your measured values to a file and compare the actual ones with the previous ones stored in the measured data file.

I don't do what you are trying to do, but something similar.

On system startup I start some scripts that run in a non-terminating loop collecting values (temperatures and Air Conditioning data) and store these values every 1 minute in a file. Nagios checks this file with a plugin I wrote.

You could write your nagios plugin to compare using the last 2 lines in your data file and give a return value to nagios based on that comparison.