Interpolation is the easiest to do. All you need is a large table of data - each column represents a specific temperature and each row a specific pressure. The data in each cell could represent a volume. It is a simple equation to interpolate between rows & columns. How to determine where phase change occurs, I don't recall....

The phase diagram for water is quite complex involving steam fractions and such. The ideal gas laws are modified somewhat to take into account steam that is partially liquid.... this is thermodynamics and its tough.

Phase diagrams for metals would be incredibly complex, with alloy percentages, temperatures, cooling rates.... that's not really thermodynamics but materials science.

Interpolation is used in industry for charts of all sorts - phase diagrams are not necessarily useful, more symbolic - the numbers from the tabulated data can be used for machine performance too (any kind of machine), and science has nothing to do with it at that point. It's purely empiracle.