This is usually due to the clock changes.

To fix:

Skip straight to 4 by using direct link (Threshold Bytes Into Bits)

1. Log into Cacti
2. Using the menu on the left Go to Graph Management → CDEFs
3. Click Threshold Bytes Into Bits
4. Click Item #2
5. Change the value to inverse of the current value  - if it's -8 change it to 8 (after clocks go back in Autumn); if it's 8 change it to -8 (After clocks go forward in Spring)
6. Click Save
7. Click Save