puzzle

The square's ascending diagonal. Also where do you get this from?

a square with a cross could work. Suppose you draw a horizontal line in each figure. The number of intersections (of the original lines) with the drawn line is the number of vertical lines. And likewise if you draw a vertical line, for the number of horizontal lines.

It's diagonal starting from bottom left. This is the beginning point in all 4 pictures, and where the line will go is determined by ratio of number of vertical lines and number of horizontal lines (it's 1, basically meaning f(x) = x)