One way:
using Measures, Plots; gr()
z = rand(100, 100)
l = @layout [a{0.95w} [b{0.03h}; c]]
p1 = heatmap(axes(z,1), axes(z,2), z, cbar=false)
p2 = plot([NaN], lims=(0,1), framestyle=:none)
annotate!(-0.2, -0.5, text("Random\n[m/s]", 9, "Computer Modern"))
p3 = heatmap([NaN;;], framestyle=:none, clims=extrema(z), cbar=true)
plot(p1,p2,p3, layout=l, margins=0mm)
