Fenix 7X Data Screen - what is this graph (bottom graph)?

Hi team

I've added a new data field to my fenix 7, however I am not sure what the data actually is?

If I long-press the data, it takes me to the solar widget however I can't for the life of me see how the data point on the watch faces correlates with any solar activity!

The graph seems random to me, and I can't seem to find any info online as to what it is!  I put the watch in full solar for a few hours, and while it charged etc, the graph showed no activity.. so not sure it is anything to do with solar, even though the long-press takes you there?

Thanks for any insight!