Click or right-click into the upper graph or press the left/right arrows to change the input function $f$.
Press 1, 2, 3, or 4 to select
an integer frequency $\xi$ or click anywhere into the lower right
graph to select any frequency.
Press r to display the real part of
the transform, i for the imaginary part,
or a for the absolute value.
Press n for
no display (which is the default). (Or click on the left of the vertical axis to switch
between these four views.) Use the up/down arrows to change the factor $L$ and use 0 to reset this factor to one. Use q
to restart the animated dots at $t=0$.
Note that the black dot isn't really showing the integral but a
scaled version of it (so that it fits) - it's rather like the
"centroid" of the orange curve.