The IsoSurface trace has a surface argument which allows you to customize the number of surfaces shown (defaults to 2 for isomin and isomax). Specifying more than 2 gives you additional isosurfaces evenly-interpolated.

It would be nice to be able to control the specific values of additional isosurfaces — for example isomin=0, isomax=50 and additional isosurfaces at values 20 and 35.

Perhaps plotly.graph_objs.isosurface.Surface can have an additional values parameter that's mutually-exclusive to count.

The invocation using the example above would look something like:

Surface(values=[0, 20, 35, 50])  # Inclusion of `isomin` and `isomax` TBD