Application: Mollier diagrams

Using this diagram as a starting point, how do I construct a similar U – T diagram?

From above plot:

Now use f1 and f2 to construct table of P at each V & T

Now plot: 1)

Finally:

Substitute: Constant P

Also: Constant T

Using both in 3):

Everything is in terms of P, V, T!

Integrate w. r. t. T & P to find U!

Using data provided in original plots we can generate U – T plot.

What if we don’t have comprehensive data?