Adding a pre-calculated 'daily average' line to graphs

[luke404]

I am trying to graph energy production data from a photovoltaic system with cacti, and I would like to add to my graphs the expected energy output.

The latter comes as an average daily production from the PVGIS database. For example, given a PV system I have twelve data points that are the expected *daily* average for every month of the year. The raw data looks like this, with example weekly, monthly, yearly graphs:

PVGIS_data.png (50.32 KiB) Viewed 343 times

I would like to draw that data points as a line over the data collected by cacti. What could be the best / easiest way to achieve this?
- afaik Cacti itself or rrdtool can't do that
- could a plugin achieve the goal?
- or should I write some software that reads the RRDs, uses rrdtool to draw the 'normal' graph, and the draws the data over that?
- whatever else?