Issue with npgraph not displaying conditional mean

[Nicolo Capirone]

I am working with a dataset composed of two panel data variables. To gauge the relationship between the two I have been running normal, polynomial, and np regressions and then displaying the results on graphs. When it come to running "npregress kernel x1 x2, vce(bootstrap, reps(100))" , Stata has no problem producing and output, but then when I try to run "npgraph", only the scatterplot is displayed with no conditional mean. I tried to look on this forum and elsewhere online, but I found not other person with the same problem as me. Is there anyone that can help me or has any idea why this is happening?

[Nick Cox]

You've fitted a surface using two covariates or predictors. As documented in the help

npgraph is available only after fitting models with one covariate.