Sometimes, when I want to wire up a few guitar pedals and connect them to my iPad, maybe via a mixer or audio interface, I make a diagram before I plug everything in. But the trouble with drawing a wiring diagram on the iPad is just the same as drawing it on paper: If you want to move a component, or change the routing, you have to erase the wires and redraw them.
There are apps made for this. Omnigraffle is one, and it’s great. But it also costs $60, which is too much just for a few diagrams.
Then I thought, what about using Keynote, Apple’s free presentations app? Doesn’t it do diagrams? It does, and it’s quite good.