I am not sure of whether I have understood you correctly, but if you
want to plot a straight line (or any function) on a colour-coded plot
(by the way, what you show doesn't give you a contour plot), then I
would do the following
set table 'test.dat'
p 'test.dat' with image, 10*x w l
Since you set the view to 0,0, it's better to use plot with image, for
which you need the data file 'test.dat'.