This applet is designed for the visual exploration of secant and tangent lines. In particular it lets the user compare the graph of a function with either the line secant at x values X0 and X0 + dX, or with the left secant at values X0 -dX and X0, or with the line secant at X0 with slope obtained from the balanced difference quotient from the line secant at X0 - dX and X0 + dX. A menu choice lets the user control whether a mouse click centers the graph at that point, moves the base point X0, or moves the point X0 + dX and changes dX. When a point can be moved by clicking, it can also be moved by dragging.
The source. is well documented, but all the files have been put into a single text file, so you will need to rename the file and separate into classes before compiling.
If you find this useful, please e-mail me at email@example.com.