Diagonal Line Draw
This might be related to "Pen Tool"
Mitsutaka Naito commented
Dear Moqups support team.
It'd be much happier if you let us know when we could see those annotation features.
Thanks so much for your feedback! Please keep it coming.
We are happy to let you know we're on the same page!
We already implemented a nifty set of annotation features that include lines (in any direction, with dedicated positioning handles), arrows, basic shapes and a drawing pen. However, it was a bit rough on the edges to our picky standards and didn't make it in this release.
In conclusion, this particular feature set will be released in the few weeks, once we polish the UI and UX a bit.