This is the relative change in the Linac velocity, which should nominally be equal to 0.71*c. The error bars are the calculated variance of the velocity during a beam pulse. Each datum is averaged over 15 consecutive beam pulses. We don't really know what the vertical scale is, absolutely, but we strongly believe that changes in this number represent (only) a change in the Linac velocity.

Offset changed from 1.865 to 0 on August 5. Data of these plot modified to reflect this change.