Is that a MPH vs. RPM chart? Those are reasonably easy, if tedious, to do by hand.
When you get to more complex formulae, doing them by hand gets very tedious, lengthy, and error-prone. There's a certain point (in the complexity of the formula) where it's easier to just set up a spreadsheet, and just where that point is depends on who is doing the number crunching. I suspect that for many of us, that point is "anything more complicated than RPM versus speed".
-soD