Jobs hanging when plotting

@charlie.hoy, I've noticed jobs hanging for a long time with the latest version of master (485c6bfd). This was for a BBH run with 54 calibration parameters and all the parameters generated in post-processing. When I killed the job after about an hour it was still creating the corner plot.

I imagine the number of parameters is causing the corner plot to not be generated in a reasonable time.

Have you seen similar behaviour (I think @gregory.ashton has)? I'm not sure what the best fix would be.