Hi Peter,

you posted the very same question two month ago and - as far as I can see - a concise answer was given. If you do not want the to see the rotated axes, then just add Axes->False to the plot options inside the epilog (giving the same answer a second time):

pArgs = Sequence[{x, -2, 2}, PlotRange -> {-3, 3}, ImageSize -> 500];
Plot[x, Evaluate@pArgs, PlotStyle -> {Opacity[0]}, 
 Epilog -> Inset[Rotate[Plot[x^2, Evaluate@pArgs, Axes -> False], -20 Degree]]]

But if you need not a graphical but a more mathematical approach you can regard any function as a parametric space curve and rotate this:

rotmat[a_] := {{Cos[a], Sin[a]}, {-Sin[a], Cos[a]}}
f[x_] := x^2
ParametricPlot[rotmat[20 \[Degree]].{t, f[t]}, {t, -2, 2}]

Regards -- Henrik