The version is 4.8.3 (could not be chosen from the list above)
It used to be possible (without currently being not able to tell with which exact version) to monitor multiple clock sensors (CPU load / clock frequency) within a single line graph. However, although the hardware platform didn´t change (upgraded from AMD Phenom X4 to AMD FX-8150), this doesn´t work anymore.

Reproducible: Always

Steps to Reproduce:
1. create a new tab by chosing File -> new tab
2. name the new tab "Clock"
3. drag from the sensor browser on the right-hand side:
CPU load -> CPU 1 -> Clock frequency
4. drop it on the topmost area on the left-hand side and chose "line graph"
5. Repeat step 3 with sensor "Clock frequency" of CPU 2
6. When hovering over the line graph, it seems that the dragged sensor can be dropped
7. Drop the sensor in the line graph 
Actual Results:  
nothing happens, the sensor seems to be lost

Expected Results:  
Two line drawings within the line graph indicating the CPU clock frequency of CPUs 1 and 2.

By dropping the second sensor in a new area and creating a new line graph (what I do not want, I prefer having the sensors in one display), I could see  that the resulting plot was in the same color. So it might be that it basically works, but you cannot tell that there are two plots as the color used (blue) is the same.
As I stated above, it used to work. When populating the line graph with sensors, the application used to chose different colors automatically so the sensors could be easily distinguished