Desmos is a web-based graphing calculator application for computers and also an iPad app. The iPad app is not web dependent and gives me the “touchable” graph that I wanted with my TI-85 back in high school. What I love about this app is that it is free and available to everyone and you can create an account (or login with Google) in order to save and share your graphs.
Click on the graph on the left to go and manipulate the parabola and the line. (Another cool feature that you can embed a graph into a webpage or an electronic document for someone else to go and view.)