Currently when setting a custom Y data range in System Monitor, the maximum allowed is 2^32-1. Setting any value above this either will not enter or lop back to -2^31+1. Problem is, a lot of values (eg. RAM usage) exceed this number (for context, this would be about 2GB, which is not a lot). This is somewhat similar to Bug 434628 but in this case involving large numbers instead of small ones. SOFTWARE/OS VERSIONS Linux/KDE Plasma: 5.12.12-arch1-1 (available in About System) KDE Plasma Version: 5.22.2 KDE Frameworks Version: 5.83.0 Qt Version: 5.15.2
Bug 433007 describes the reason for the limit.
After reading a bit on Bug 433007 as well as some other related bugs, maybe it would help if there was a separate box for the scale (like x1k, x1M, x1G, etc.) to circumvent the limits that the spinbox has.
For Plasma 5.23 we now support selecting which unit to use for the Y range. This should make it a lot simpler to select the right range.