I want to say that in a plot, not all the points are drawn. Instead we draw some of them with a specific interval (k). For example, with k=3, points 1, 4, 7, 10, 13, etc. are drawn. I think that for k=2 the correct way of saying that is to say "every other point is drawn." But for other k I don't know how to say that. Maybe something like "every other three point is drawn"? How should I say that?
"For every third point" usually refers to a sequence of 3,6,9,12... "Every fourth point" is 4,8,12,16... For the sequence 1,4,7,13... you should say "every third point after the first". Or you can say, "the first point and every third point thereafter".
In computing, the interval between drawn points would be called the increment.
noun: a usually small amount or degree by which something is made larger or greater
Increments are always positive, but the word decrement is available for negative step values.
I don't know if it is grammatically correct, but I've heard people use the word as in the following example.
"Plot points 10 through 100 increment 20."
This means that points 10, 30, 50, 70, and 90 should be plotted.