In this demo, we show the flux across a curve C(t). The window "The
unit normals" shows the vecor field V(x,y) and the curve C(t); the red
vectors are the unit tangent at certain points on the curve. The second
window, "V(x,y) at C(t)" shows the vector field V(x,y) evaluated at
(x,y)=C(t).
In the third window we see a graph of the function that gives the value
of the dot product of V(C(t)) with the unit normal at C(t). The flux
across C(t) is then equal to the (signed) area under the curve, and you
can read its value off the readout "Flux".