The plugin just shows heights; are these heights just calculated from temp-1000h, temp-900h
They are calculated using the gh-1000h, ... (geopotential height)
Regardless, if it is valid, is there any data missing or is it still possible to calculate the points on the diagram?
Yes, it's possible to compute those values and it's called a weather model ;)
Can the model elevation be used for the surface data? (The builtin sounding shows the model elevation, but perhaps the plugin doesn't have access to this?)
The plugin has access to the same data. But again I don't think surface can be used (to be verified)
The Meteoblue temperature and dew point lines always starts at 1000hPa, and it also shows a horizontal surface pressure line. All the other examples start at the surface pressure, whatever that is for the particular location and time.
I think some models provide a temp-100m, ... which could be helpful. This is not part of the data provided by windy.
Maybe @marekd knows more?
Thanks for the screenshots, the ensemble graph on ECMWF gave me the idea to overlay multiple models on the sounding.