[quote author=Fu-Kwun Hwang link=topic=907.msg3402#msg3402 date=1233227368]
You can use AnalyticCurve to draw curve directly.
Since you used
double u = _view.waveFunction.evaluate(x,t);
to evaluate it's value in your code.

You can calculate u2=_view.waveFunction.evaluate(x,t-ddt);
And you can get du/dt with [u][color=red](u2-u)/[/color]ddt[/u] if [u]ddt[/u] is small enough!
Just calculated it by definition!


it was a brilliant idea/method but it didn't solve the problem :( It is because the code above is for the n = 50 points that is spread out in the x-axis, the little points.

original code from paco and wolfgang seems to use the %string% method to draw the nice function.
guess it is difficult to get du/dx =  k*cos ( k*x - w*t) from a string u = sin ( k*x - w*t).

i go try your idea but on a separate loop.