This works:
Show[ graph[0.72, 1, 400, 300, 700], Axes -> False, Frame -> True, FrameLabel -> {"mu", "x*"}, PlotRange -> {{0.72, 1.02}, {0, 1}}, AspectRatio -> 1 ]
The -> in the options stand for Rule, which is different from RightArrow in your code.
->
