Descartes is a small software which you can use to draw graphs of mathematical functions.
Mouse right-click on the graph background offers various options, some of which are obvious - zoom in and out, centre, and reset graph limits to the original : +/- 5. The non-obvious options will display a chord on a function, showing the 'rise and run', and the slope of the chord as 'rise over run'.
With appropriate zoom in, continuous functions become locally straight, and the chord slope becomes the function differential coefficient. The other options enable the user to select where (what x value) the chord is shown at, or to use the next function, if several functions are being displayed.
