You can change the phase by holding ctrl-shift and dragging the circle. If you want a negative value just use the absolute magnitude with the component in antiphase.

There's a triangle wave preset already available. Right click on the page to select it. It uses values: 0.75, 0, 0.083, 0, 0.03, 0, 0.015, 0

Very helpful - thanks.

It uses the phase. The 0.75 is 180 deg out of phase (blue line of circle points to the left) compared to the 0.083. Compare to the square wave, as well.

Yes, that was the only thing I was missing. I'm tempted to adapt this UI though, it's lovely design.