One of the most important diagnostic plots for future jointcal development would be a way to look at what's happening with the models and outlier rejection at every iteration of the fit.  That could be done by instrumenting the code itself with "immediate" interactive display logic, but we think this would probably work better by having an option to dump the necessary data to make these plots after a run (this of course must work even if jointcal fails in an unexpected way; that's sort of the point).  That's in large part because we expect it to be rather a lot of diagnostic output, and being able to filter it different ways will be important to keep the display from being overwhelming.

This would be an ra-dec scatter plot (or possibly an overplot on a displayed image), with points for each source transformed by its WCS at that iteration and a small line segment showing where it would land with the WCS model for the next iteration.  Reference objects should appear too (with some way to identify them, of course), as should all outlier-rejected sources (including those rejected on previous iterations).  Being able to show (distorted) CCD outlines given the WCS for that iteration might be useful, too.

I could imagine that being too busy if it showed all sources at e.g. tract scale; we would probably want to be able filter by random downsampling, by magnitude, by signal-to-noise, by RMS scatter, or by something else that's in the associated SourceCatalog but perhaps not the jointcal-specific data structures we're dumping.  I could also imagine us wanting to just have one dot for each object, even if this only lets us see which objects are outlier-rejected, and not how they'd move.