Found the answer: Under More Layers, Display Isolines, and select Geopot. height.
Now the equivalent altitude will be displayed as isolines while keeping the pressure constant.

So yet again, Windy already had the facility I wanted. It's just too great!

With the wind display I can move the slider to show lower pressure levels, but the isolines stay with the surface pressure.

Suggestion: When move above surface pressure, isolines show the heights.
Alternative (and perhaps more intuitive/seamless coming from the surface): Change the slider to be height driven, with the isolines showing the pressure at the given height.

These would enable us to tell what's going on at high altitude.

Fantastic thanks. Yet again, shows what a great product Windy is already!

In your wind charts the highest colour in the scale is for 100km/h, 60mph etc. That's too low, especially when looking at high altitudes and the jet stream where it's important to see the difference between 100km/h and 200km/h or higher. Could the scale be extended always, or extended when showing an altitude above surface?

Be fantastic to have a vertical view of temp/dew point running of the model from a chosen point. Ideally it'd be a popup or a section of the page, so that as you moved to future time steps the tephigram would update in sync.