I want to draw mathematical pictures like graphs but just simple ones like a quick sketch of an exponential decay crossing at a point marked A or something like that, and I want to draw a picture of a diagram of some equipment, like a lens and a light ray being sent through a pinhole and projected onto a screen. Diagrams like this:

But so they look more like this:



and:

Is there any software on Ubuntu that'll let me do this easily and accurately?