The timeline was about right, but I was surprised by the relative frequencies.
I had to use separate graphs because the scale was dramatically different. 'Okay' at its peak is 1000 times more common in books than 'okeh' at its peak. The difference might not have been quite so dramatic in handwritten notes or quality control markings, but those aren't archived.
The big point is that the word itself, regardless of spelling, was NOT considered okay/okeh for publication until recently. Like everything else in the world, 'okay' went exponential in 1974.
The current icon shows Polistra using a Personal Equation Machine.