x=0:.1:5;
y=(x-2).^3;
patch([0 5 5 0],[0 0 4 4],[.75 .75 .75])
hold on
plot(y,x,'LineWidth',2)
hold off
If you do prefer to have a transparent bar, then adjust the FaceAlpha property. Just note that this will also cause a slight change in the color of the patch.
x=0:.1:5;
y=(x-2).^3;
plot(y,x,'LineWidth',2)
patch([0 5 5 0],[0 0 4 4],[.75 .75 .75],'FaceAlpha',0.25,'EdgeColor','none')