How to interprete the comparisons diagrams

For each station and year a diagram is presented which contain an intercomparison between SWD (Short-wave downward, GLOBAL radiation) on the x-axis and SumSW (sum of Diffuse radiation and Direct radiation on a horizontal plane) on the y-axis. SumSW was calculated using the  BSRN-Toolbox (Qualitiy Check, comparisons option) see diagram to the right.

The data are vizualized via gnuplot.The displayed linear regression (green dotted line) was calculated via the gnuplot fit command.

In an ideal world SWD and SumSW should be identical. All deviations from identity denote errors.