XMM-OM Serendipitous Ultra-violet Source Survey

I was unsuccessfully looking for a VO interface to the Chandra Source Archive in the various VO registries when I found this catalog with the VO Desktop software. I was curious: 6 band photometry; the three UV bands lie within the GALEX NUV. So a little simplisitic messing around to make the plot above is captured as a recipe for UV goodness with the SUSS:

1. Downloaded entire FITS table (750k sources) from within VO Desktop.
   
2. Loaded into TOPCAT. Created color (U-B, etc) columns.
3. Plot is the UV color (189nm - 268nm) as a function of B-V (nearly Johnson; from the XMM-OM). Nice pair of probably luminosity class sequences.
   
4. Plot symbols are rendered using the magnitude errors in the UV bands as auxilliary axis. The symbols become more transparent the larger their errors. Sources with magnitude errors > 0.25 mag are effectively rendered invisible.