  The diversity of vortex melting and solid-solid transition lines measured in
different high-T$_{c}$ superconductors is explained, postulating a unified
order-disorder phase transition driven by both thermally- and disorder-induced
fluctuations. The temperature dependence of the transition line and the nature
of the disordered phase (solid, liquid, or pinned liquid) are determined by the
relative contributions of these fluctuations and by the pinning mechanism. By
varying the pinning mechanism and the pinning strength one obtains a spectrum
of monotonic and non-monotonic transition lines similar to those measured in
Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$%_{8}$, YBa$_{2}$Cu$_{3}$O$_{7-\delta}$,
Nd$_{1.85}$Ce$_{0.15}$CuO$%_{4-\delta}$,
Bi$_{1.6}$Pb$_{0.4}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ and (La$%
_{0.937}$Sr$_{0.063}$)$_{2}$CuO$_{4}$
