I'm sure you've already seen what the following settings do:
- Tools->Custom Graphs->New...
- Series 1: Ext. Temperature (Series 2 & 3: None)
- Style: Filled (or your choice)
- Data: Anchored Min/Max (and Avg if desired)
- Period:Month
It
would be nice to have Min, Max, and Avg different colors, but the length of the bars is pretty explanatory. But a Line graph might be much more difficult to read, in my humble opinion, even if different colors were provided.
I haven't read up on the "synthetic" thing.