It looks like the SRX sessions graphs are kind of broken with the higher end SRX's that have multiple SPUs/FPCs. I saw there was a JIRA bug (OBS-2101https://jira.observium.org/browse/OBS-2101) opened a long time ago that observed the same issue. The way these are being graphs right now is via the generic graph module. But the logic to graph this correctly needs something a bit more custom. The code needs to iterate through all the available SPU's/FPC's, potentially on both nodes if it is a cluster, and graph all of the values. Each SPU/FPC has its own set of limits as well.
I was wondering why some of my SRX's worked and some didn't. It turns out those that do not have an SPU in FPC 0 will fail to return any results. And those that do have an CPU in FPC 0 might be returning partial results because the other SPU's/FPC's are not being graphed. I have attached debug polls from an SRX4200 that works, and an SRX3600 that doesn't. The SRX4200 starts at FPC0, but the SRX3600 starts at FPC7, so the default index of 0 doesn't work here. Even if the code took the first item in the array, it would still be wrong because it needs to also consider FPC11 and FPC12, not to mention the corresponding FPC's on the second node of the cluster (indexes 33, 37 and 38).