Quiver plot.

  QUIVER(X,Y,DX,DY) draws little arrows at every (X,Y) pair in

  matrices X and Y. The (DX,DY) pairs in matrices DX and DY

  determine the direction and magnitude of the arrows.



  QUIVER(x,y,DX,DY), with two vector arguments replacing the first 

  two matrix arguments, must have length(x) = n and

  length(y) = m where [m,n] = size(DX) = size(DY).  In this case, the

  arrows are the quads (x(j), y(i), DX(i,j), DY(i,j)).

  Note that x corresponds to the columns of DX and DY and y corresponds 

  to the rows.



  QUIVER(DX,DY) uses x = 1:n and y = 1:m. In this case DX and DY

  are defined over a geometrically rectangular grid.



  QUIVER(X,Y,DX,DY,S) and QUIVER(DX,DY,S) apply scalar S as a scale

  factor to the lengths of the arrow. For example, S = 2 doubles

  their relative length and S = 0.5 halves them.



  A final trailing string argument specifies linetype and color using

  any legal line specification as described under the plot command.



  For example, try

     xord = -2:.2:2;

     yord = -2:.2:2;

     [x,y] = meshgrid(xord,yord);

     z = x .* exp(-x.^2 - y.^2);

     [px,py] = gradient(z,.2,.2);

     contour(x,y,z),hold on, quiver(x,y,px,py), hold off