I'm going to refer you to my Drawing a Function and a Tangent to a Point Tutorial that you may or may not have done at this point to start with.
We're going to cheat a little bit though, make the two points set really close to each other.
Select your tangent (okay it's a secant I know) and go to Display -> Trace Line
Select the two points like this and move them to the left end of the graph that you see and then go to Display -> Erase Traces
Go to display -> animate with the two points selected and WOWIE.
The trace will appear in whatever colour you decide to make your tangents and you can even change the colour of the tangents part of the way through.
