My Year 12 (age 16-17) students have been studying coordinate geometry recently. I have been advising them whilst they are studying this topic that it can be very helpful to see good diagrams. I created the above slideshow to provide them with diagrams for a set of questions assigned to them and also to show them examples of the syntax to use for the excellent Desmos graphing calculator. Clicking the image on each slide takes you to a Desmos page with the graphs.
Registering with Desmos (free) gives you the ability to create, save and share pages (Facebook, Twitter or permalink).
Several students have said how much this has helped them.
For further examples see this post.