Within that graph following two values (over ALL satellites) are compared:
max(storj_stats_storage_free_capacity_estimate_bytes) - green
statistical estimate of free storage node capacity, with suspicious values removed
sum(storj_stats_storage_remote_bytes) - blue
number of bytes stored on storage nodes (does not take into account the expansion factor of erasure encoding)
Because the expansion factor is not taken into account i called that value stored customer data. Of course its a bit comparing Apples to Eggs, but thats the nearest i could get to, without modifying (and thus possibly falsifying) the reported data.
Yes you are right, so it is important to understand the Values you are looking at. In my opinion it is still useful data because i, as an SNO, am interested to see if/how the network is utilised, which is reflected by those numbers.