Hi @eyebp,
No, you cannot insert a trace on the diagonal. help(go.splom.Diagonal) illustrates that the only update of the scatterplot matrix on its diagonal is to set diagonal_visible=False (True is the default value).
go.Splom class has a special definition (see the paragraph Scatter matrix (splom) with go.Splom in this tutorial https://plot.ly/python/splom/ . That’s why the user cannot access the subplot cells.