I didn’t manage to use @gustaphe’s elegant syntax but the brute force below works, FYI:
using Plots; gr()
l = @layout [grid(1,2){0.3h}; b{0.5h}; c{0.25w} d{0.5w} e]
p1 = p2 = heatmap(rand(100,20))
p3 = scatter(rand(10,2))
p5 = heatmap(rand(200,20), colorbar=nothing)
p4 = p6 = plot(frame=:none)
plot(p1,p2,p3,p4,p5,p6, layout=l)