From the documentation, you can use SI unit prefixes but I seem unable to get millibar.
I tried m-bar, mbar, m bar, milli bar, millibar, milli-bar all to no avail.
If I try bar/1000 i get hecto pascals (hPa) instead.

Am I just doing it wrong or is it not possible?

edit: Also when using ppm the output symbol is "/1000000" rather than "ppm" which would be preferable in my opinion