These are plots of the recorded breakdowns in the station scores--the plot on the upper left labeled "Overall" shows the simple plot of how many stations are rated 10, then above it how many more are rated 9, then how many are 8, and so on down to 0, with the top line being the total number listed on Plugshare, basically just a stacked plot of this:
If you look at the upper left plot, you'll see the bottom band on the right is about 330, then above it the second band ends at (334+90), the one above that is (334+90+76), and so on--so you can see both the overall growth, and also how the different band of Plugscore contribute to that.
The upper right plot is the same data, but normalized--so instead of simply saying there's 334 stations that have a score of 10, it says there's 334/629 = 53% of the stations that are currently rated as ten, so you can see more clearly how each band contributes to whatever the current total is without the change in total swamping the differences.
The lower left plot is the same method as the upper right, but excluding all of the "0" rated stations (so the total is 629-44, not 629 for the most recent data point). Stations hang out at 0 until enough people come by to rate them, which can mean a working station in a low-traffic area waits a long time at 0 before suddenly jumping to its proper score--especially with fewer people traveling for holidays during the pandemic. Excluding those gives some useful data, so I also plot the stacks this way.
The final chart is a weighted average. Basically, it multiplies each tier's value by its relative frequency in the data set (e.g. 10*0.53=5.3) and then sums those values to give the "average" station's plugscore for the network as a whole. I calculate this both for the entire network (the blue line) and for the network excluding those stations rated 0 (the green line), which again as noted are mostly just stations which haven't gotten enough reviews to be rated yet. The red line is the percentage of stations with a non-zero plugscore below 5 (e.g. 3 of 629 in my 6/11 data point). This number of "flaky" stations being low indicates stations are generally in good working order and any persistent issues are being resolved and not building up.