How can I get the tangent of a drawn graph at a certain point?

[Verena]

I've drawn a graph with output data of a file. Now I like to get the tangent of that graph at a certain point. How can I calculate that and afterwards print it in the graph?

[GabbyG]

The tangent ist the derivative of the curve in that point. You can try to use a numerical derivative vi or keep it simple an approximate with a line through the adjacent points.
Gabi

[Gianluca77]

Sorry, but I have the same problem, I have an ellipse and I want calculate the tangent line in a point  appertain to ellipse.

I know the center point, max radius and minimum radius of ellipse!