What Gary said...
is very true. The phase diagrams are basically a graphical plot of tables of values, which as he noted, can be found in the back of any standard thermodynamics textbook (e.g. see entries for subcooled liquids, etc.). There are many "flavors" of custom-designed software routines floating around out there at various labs, including here at UH SOEST-HIGP, though I tend to "do it the old-fashioned way" (i.e. either by interpolation or by assigning it a grad student :-)
Alex R. Blackwell
University of Hawaii