How to contour Gridcells in Julia?

Question

I want to plot some areas on a world map and contour them because some of them are overlapping each other.

First attempt: contour plot

function scale4plot(A,f=100)
  resize(A,f) = [A[i,j] for i=1 .+(0:f*size(A,1)-1).÷f,j=1 .+(0:f*size(A,2)-1).÷f]
  B = resize(A,f)
  ax,ay = ( (axes(B,i)./f).+0.5 for i=1:2)
  ay,ax,B
end

plot(Plots.contour(scale4plot(dplot[:,:,i]), w=1, linecolor=:black, fill=true )))

But I couldn't figure out how to plot a map over it.

Second attempt with GMT: GMT plot

G2 = mat2grid(dplot)
grdimage(G2)    
coast!(region=[0 360 -90 90],show=true)  

But here i couldn´t figure out how to outline the areas or decrease the opacity. I also couldn't figure out how to plot multiple plots in one with grdimage.