If you hover over a node, it shows the full report from the account metadata of @fullnodeupdate . Each dot from left to right represents an overall ranking of this node for different areas, that make more sense if you read the full reports from fullnodeupdate. I sort servers by just adding up all their ranks. A rank of -1 signifies an error (and the overall sorting score for that server is heavily penalised for errors).

The colours are a simple gradient from light green (best) to dark orange (worst) with red for errors.