Useful software for notes and graphs in economics?
I've been trying to find a solution for this same question.  As mentioned by others, most of the solutions are a bit too time consuming to do on the spot if you're taking notes, so I still think it's easier to draw out the graphs by hand on the spot.  

There's a good post referencing some of the different software options, along with how to do this with R and ggplot if you're interested on Andrew Heist's blog 

Also, see Create simple graphs without functional forms

Among these options, I'm trying to learn to do everything with TiKZ, but it's a work in progess...