I use KDE 4.8.2 on an Opensuse 12.1 system. 
I have several ksysguard sheets form the past with multiple sensor items per graphical cell on the worksheet. These sheets work as expected. 
However, when I now want to drag and drop several sensors (e.g.) "clock frequency" from CPU 1 and CPU2 and CPU 3 to one and the same graphical display cell on the working sheet only the first one is accepted, but not the following sensor items. Dropping a second or third sensor over one and the same graphical display cell within the ksysguard grid has no effect. 
This  
  

Reproducible: Always

Steps to Reproduce:
1. Open a new worksheet with several lines and columns, i.e. several display cells . 
2. Drag and drop the first sensor item from the sensor list to the first cell. Choose the graphical display style (e.g. line graph).  
3. Try to drag and drop another similar sensor to the same first display cell. 
Actual Results:  
The first sensor item is displayed in the chosen cell, but not the following ones.  

Expected Results:  
All sensors dropped in one and the same display cell should be shown in different colors.