use levitus_climatology

contour/fill/pal=grads/lev=(-INF)(-20,-10,5)(-2)(INF)/line=1/SPACING=0.1 temp[k=1]-30
message

say Problem: labels are drawn for last contour lines, here level=-2
contour/fill/pal=grads/lev=(-INF)(-20,-10,5)(-2)(INF)/line=1/SPACING=10 temp[k=1]-30
message