Wow! Yes, that's what wished, and now I have got it. Thank you so much for helping!

I wonder if it is possible for user to choose to show only the graph of first derivative or the second derivative, or both. If not, it would be good just to indicate in words somewhere that the white line is the the graph of first derivative, and the red is for the second derivative. As teachers, we could see easily that the colors of the lines are already an indication of this, but many students may not be able to see this.

Once more suggestion: white line and gray background do not give sufficient contrast, especially when the applet is projected onto a screen in a classroom during the day. A black line should be better (then the color of the dot moving on the blue curve should be changed accordingly).

Thanks again!