Interactive Axes allows you to plot grid coordinates onto an image of your choosing. Simply Take a picture of a curve with your built in camera, or select an image from your photo library, and plot points on it.
The coordinate data is displayed on screen for you to copy down and work out the equation of the curve you have plotted. You can also email yourself a copy of the coordinates as a file which can be opened in a spreadsheet application.
Once you have plotted your curve you can move the origin point of the grid around to see how that affects the coordinates. Or replace the image to compare two similar curves.