The sensors plugin seems to be too wide (by 2px) compared to netload and other plugins with graphs. It also seems to sit too high/low by 2px.
The masses are always wrong. Please file bugs against all other panel pluginbs that do not adhere to the panel plugins guidelines http://wiki.xfce.org/dev/hig/panel-plugins : box width of 0, spacing of 2 between children, i.e. left and right to the next bars. Please correct me if I am indeed getting it wrong.