One of the KDE Plasma features I use almost in daily basis is the MouseMark.
Go to your settings, in Desktop Effects, Enable the MouseMark (you can change the settings of it in the button in front of it). Then press Meta+Shift and move your mouse to doodle, or press and release Meta+Shift+Ctrl to define beginning and end of an arrow (it will draw the rest.
Interestingly enough, it have been in Plasma for the past 16 years:
https://github.com/KDE/kde-workspace/tree/master/kwin/effects/mousemark
Interesting, I think there was a Firefox addon way back that did this and mouse gestures.
What do you use it for?