Hi,

I am trying to do a surface plot on data with missing/NaN values but the camera angle that I want gives me the error: “GKS: Colour index is invalid in routine SET_FILL_COLOR_INDEX” and the coloring is all black.

Using pyplot as backend does not result in this unexpected behavior.

Find a MWE that produces this behavior below:

```
using Plots
using LinearAlgebra
gr()
M=10
mat = fill(NaN, M,M)
for i in 1:M
for j in 1:M
if i > j
continue
else
mat[i,j] = -1 +1/i
end
end
end
mat[1,:] .=-1
mat[I(M)].= [-1 + -0.05*i for i in 1:M]
wireframe(1:M,1:M, mat', camera=(70,30))
surface!(1:M,1:M, mat', alpha=0.8)
wireframe(1:M,1:M, mat', camera=(270,30))
surface!(1:M,1:M, mat', alpha=0.8)
```

The first camera angle works fine, the second produces the error.

Is this a bug or is there a reason for this behavior?