The reason why there is some discontinuity is that there are more broad lines. Out of 4 HgI lines in the red, I have identified two HgAr lines, at 690 and 708 are obviously broader then nearby Neon lines. The flux for other lines is too low to make firm conclusions.

I have been using the line at 690 in my solutions. I will have to artificially narrow that line (as in PIPE2D-561). I will close this ticket when I have done so.

Talked with Jim this morning (Mon, April 27, 2020). Jim suggests:

1. Checking if this is allowed transition to the ground state i.e., if this is the same effect as seen in Argon lines. To do this, calculate the energy of the transition from NIST line list, transform to wavelength, and then compare with a list of known transition lines from the ``literature''.
2. Compare the widths of Hg lines in the blue and see if only some lines exhibit this new broadening. 

I will close this ticket and put the suggestion by Jim in new tickets for future work.