You can start by looking at the "Diagnostic Information" section in the "Postprocessors" help topic. (I'm using an ancient version of EC, so your help may look different.)
From that, you can see that there are "diagFiles" that are created for a job, which contain the diagnostic information for each job step. (To check out what I mean, do
ectool getJobDetails someJobId | grep diag
That should tell you the names of the diagFiles that were created in the job's workspace for the various job steps. In my version of EC, the propertyNames are diagFile and the values are "diag-stepId
.xml". This is just for your preliminary investigation, as there's probably a better way to get this information for a production environment.)
The help topic mentioned earlier will tell you the format of the diagFiles, which you should be able to get for your dashboard.