OK, I've not found the answer in two years so now I'm admitting defeat and asking. I know N1MM can do this because I've seen screenshots however there's nothing in the 300 page manual that tells you how to do this.
So what is this elusive thing?
In the digital audio window, how do you change the spectrum view to waterfall when using the inbuilt MMVARI?
Anyone any ideas how to do this?