I assume you have the base image and the line data prepared. To plot a line above the image, I suggest to do the following.
Here, I used the same data for the base map and the line above for example. See following code:
%%base image
[X,Y,Z] = peaks;
figure
contour(X,Y,Z,20)
%%add line above
line(X,Y,Z+15) %add height to Z
view(3) % change the position of the viewer
This code will give you
