I tried the CDEF to see if this would generate the graph that I initially
expected and (of course) it does. As you suggested the total value is still
calculated with VDEF.

It would be nice if I could draw vertical lines after each interval from
height=rate/value to the x-axis. That way the graph would look more like a
histogram which might help making the correct interpretation more easy. I
looked at VRULE, but that's not going to help here as far as I can tell.

I already established for myself that the calculated total value matches
with what the meter shows at different times. So I am pretty sure everything
is ok now. Everytime I checked, the last stored value (as seen in rrdinfo
output) also matches with the meter value, so rrdtool is working with the
correct values and everything that I think comes out wrong is all just me ;)
To be 100% sure I will do some write-downs of the meter value and create
some graphs with the same intervals as the write-downs. Things (the total
value) should match then.

The gas meter itself updates in real-time, however the smart meter (from
which I get the data) only 'asks' the gas meter once every hour for the
counter value at that time. This can't be influenced, so as far as I can
tell a step size smaller than 3600 doesn't make sense in this case. Because
the smart meter also gives me power data and does so every 10 seconds, I
actually read (and store) the gas counter value and power values also at a
10 second interval. Therefor I decided to use the LAST CF for the gas
counter. The MAX CF would probably have the same effect since the counter is
only going up.

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