contour_plot(P(x,y)~x&y, domain(x=0:40, y=20:60), contours_at=seq(20,50,4))
Warning messages:
1: The following aesthetics were dropped during statistical transformation: z
ℹ This can happen when ggplot fails to infer the correct grouping structure in the data.
ℹ Did you forget to specify a group aesthetic or to convert a numerical variable into a
factor?
