Unlikely that solar activity would affect only one transponder unless it is weak and near threshold.
The meter is a feature that I developed for STBs called a Signal Quality Histometer. It is a visual representation of the signal quality for the past xx seconds. The purpose of the histometer is to help the installer optimize signal quality by showing the increase and decrease of the signal quality over time as adjustments are made.
The intermittent dropped bar indicates that the transport stream was unable to be decoded during that time. This could be caused by encoding or decoding errors due to incompatible data, interference or a failing component.
Since other services on the satellite were being received without error, it is likely that it was interference only on that frequency, encoding error in the source (or at the uplink) or a decoding error in your receiver.
If this happens again, record the transport stream and use TS analyzer software to determine why the decoding errors are happening. Also be helpful to view the signal with EBSpro or TSReader to view the signal and the transport stream simultaneous and live.