We use a holographic dual model for the heavy-ion collision to obtain the phase diagram of the quark-gluon plasma (QGP) formed at a very early stage just after the collision. In this dual model, colliding ions are described by the charged gravitational shock waves. Points on the phase diagram correspond to the QGP or hadronic matter with given temperatures and chemical potentials. The phase of the QGP in dual terms is related to the case where the collision of shock waves leads to the formation of a trapped surface. Hadronic matter and other confined states correspond to the absence of a trapped surface after collision. In the dual language, the multiplicity of the ion collision process is estimated as the area of the trapped surface. We show that a nonzero chemical potential reduces the multiplicity. To plot the phase diagram, we use two different dual models of colliding ions, the pointlike and the wall shock waves, and find that the results agree qualitatively (based on JHEP05(2012)117).