Haahahahah good observation, looks like one of the curves on the computer being used by the engineer in their promo videos
I'm using Audio Hijack by Rogue Amoeba! It's Mac-only, unfortunately, but i'll be damned if it hasn't made the entire journey of EQing headphones easy as pie, along with Pro-Q. Another really great tool is this plugin called CanOpener that's actually supposed to be a crosstalk/speaker simulator, but I use it internally-bypassed just for it's awesome visualization of the L/R stage relative to frequency (see attached picture, it's super intuitive). Super helpful as a visual aid when trying to pin down stereo imbalances and controlling for the inevitable variation that comes about from headphones' placement on the head.