I'm looking for a simple tool (preferably online or at least that works on Linux) to draw stencils. When teaching and creating notes for students, I often feel that it would make understanding in some topic much easier if I could draw stencils like the one below. But doing stencils like that by hand (with tools like Gimp of by typing code) can be very cumbersome.
Is there any tool specifically designed to make things like this easy?