[chemeng]

Don't know if it's what you're referring to, but psychrometric charts are always fun, and any of the thermodynamics charts (Pressure-Enthalpy being an example). The best part? You say the "age of slide rules", but these are still used today.

[rckoepke]

Not psychrometric...the ones I'm thinking of look exactly the same as this, but allow you to solve a system of equations by drawing a straight line between points on two columns, thus making the intersection of the line with the other columns your "answer" for those variables.

The scales are sometimes very strange, in order to achieve this effect.

[chemeng]

Ah, nomogram then?

[rckoepke]

Living up to your name! Thank you so much.