A workaround using layouts:
using Plots; gr()
l = @layout [a{0.95w} b]
cmap = cgrad(:thermal)
p1 = plot(rand(10), xlabel="x axis", ylabel="y axis")
p2 = heatmap(rand(2,2), clims=(0,1), framestyle=:none, c=cmap, cbar=true, lims=(-1,0))
plot(p1, p2, layout=l)