I just requested similar functionality for the Shape tool when drawing lines, and thought it would also be useful to have something similar in the Draw tool. I've raised a separate feature request because this one is for a different tool.

It would be good in the draw tool if holding the Shift key while drawing constrained the line to be horizontal or vertical.

In my use case people will load images and might want to underline things on the image.

Currently there is no way I can see to draw a perfectly horizontal line.

If in the draw tool you could hold shift and draw a perfectly horizontal line it would make this really easy and intuitive for the end user.

PS. To make the functionality more consistent with the Shape line tool, you could also add Ctrl as a modifier to the Draw tool to constrain the line diagonally.