Please let us now about your device and OS version for it’s perfectly straight when I try that on my iPad Pro 12.9 2018 iPadOS 13.2. In full view as in preview.
Hmm, it’s very odd. The ruler is entirely in the realm of Apple’s frameworks. We have no. control over that.
Does the line look horizontal in the drawing view? Ie is it just the inline version that looks odd? Is it just a question of a small image being blown up, so the antialiasing of the line causes a strange effect?