What the graph represents is the "auto" that you're assigning it. It is the conversion from cpu temperature to fan speed. As the cpu heats up the fans spins faster, depending on the settings you have set in that graph. If the App Center is not distinguishing between CPU and SYS_FAN2 on your system then there's not much I can do to help you.
The motherboard I have is the Z87MX-D3H. On my system the App Center does break it down into CPU FAN & OPT, SYSTEM FAN 1, and lastly SYSTEM FAN 2 & 3.