Thankyou very much for the detailed reply.
The y-axis is labelled Messages, so what this really means is Messages/s? I did wonder that, because in older posts (eg here, here, here) the axis is labelled as either Messages/s or Message Rate. This made me wonder if this version was explicitly just some absolute number instead of a rate. Logically it looks like a rate, but then I wondered why it was explicitly labelled as such on older posts but not on this more evolved version. I think I’ve been over-thinking it 
I guess this is essentially capturing the numbers from the top-right of Skyview which shows Total Aircraft as a number and Messages as a number/sec. This updates every second, and the high resolution rrd capture is recording those each second.
At first glance it seems like an alternative interpretation could be that for any given message rate, the LNA during the red period saw more aircraft than the LNA during the blue period. This is because for a given value up the y-axis, the red plot is further along the x-axis.
Could this not lead to the conclusion that the LNA in the red period was performing better – it was able to see more aircraft for a given message rate, perhaps able to present cleaner decodable messages? The fallacy of interpreting it this way though isn’t immediately clear to me, which bothers me. It doesn’t work higher up the x-axis because there are no plots from the red period showing there.
Thanks for the second worked example. It’s very striking and really brings home the differences sliced across all these different metrics.
I have a build document here so that future me, and a mate who also feeds, is able to do a fresh install of PiAware from scratch and get it all set up. I’ll add this note to the graphs part so that the max res data is being saved all the time.
Thanks for the commands, I currently back up the graphs manually, and update following the instructions on its github page. I’ll work through your script and the 3m → 1m tweak so I’m clear on the best approach to save not just the graph data backups but the scatter data but allow graphs to update and be happy. Is the 3m → 1m tweak going to affect graphs1090’s graphing?