Double-click the ruler to snap between vertical and horizontal
Double-tap the ruler to snap it to either 0° or 90°.
If the ruler is at 0°, rotate it to 90°.
If the ruler is at 90°, rotate it to 0°.
If the ruler is at an angle, snap to either 0° or 90°, whichever is closest.
Zach John commented
in fact, I prefer an idea that other note-taking apps leg: good notes 5) have on iPad. They have an ion, shapes, which when selected, causes horzontal lines drawn to be straight lines, and vertical lines drawn to be vertical. Circles drawn, to be perfect circles and squares to be good squares .therefore, a ruleris not required for these simple drawings suggest to implement this simple idea when drawing most shapes.