After finding some time, I finally managed to solve this. I just needed to read the chart wiki a bit more careful. The result is now like this:
The snippet from the graphs.conf as follows:
[[[outTemp_avg]]]
zIndex = 3
name = Mittl. Temperatur
color = orange
observation_type = outTemp
aggregate_type = avg
[[[[marker]]]]
enabled = true
radius = 4
What was needed, was only the definition of the observation_type and aggregate_type.
However, one issue is remaining. I tried to scale the the yaxis for the temparture by:
yAxis_min = -5
yAxis_max = 40
This leads to the graph above. I think that yAxis_min does not accept negative values. Correct?
If I change to:
yAxis_min = 0
yAxis_max = 40
Then everything works as expected.
Any idea how to solve my remaining issue (if possible)?