there is one, you just gotta hold shift

If using a computer or laptop, you can hold shift to make a straight line. But of course, most people probably use phones or tablets, and what if you want a perfectly horizontal/vertical/diagonal line? What do we do there?? THERE SHOULD BE A TOOL!!! I love this idea SO MUCH!!

You can usually hit shift from the last point you drew from to make a straight line! I know that sounds confusing, but it's actually pretty simple; just put a random dot somewhere and hit shift. Then you should be able to click/tap a random spot on the screen and it will make a straight line. Probably ask the devs for a better demo though, I'm really terrible at explaining things. Hope this helped though :)