function vector = draw_arrow_2d ...
     ...
       (x1,y1,dx,dy) 

%--------------------------------------
% Generate coordinates for plotting
% a five-point arrow starting at the point
% (x1,x2) and ending at the point (x1+dx, y1+dy)
%
% SYMBOLS:
% --------
%
% dx, dy: arrow vector
%
% vector(i,1): x-coordinate of the ith point
% vector(i,2): y-coordinate of the ith point
%              where i=1,2,3,4,5
%
% angle:  angle of arrow tip in radians
%
% tip: length of arrow tip sides
%      as a fraction of the arrow length
%---------------------------------------

  angle=0.3; tip=0.50;

%--------
% prepare
%--------

  x2 = x1+dx;
  y2 = y1+dy;
  cs = cos(angle);
  sn = sin(angle);
  dxi = -dx;
  dyi = -dy;

%------------
% first point
%------------

  vector(1,1) = x1;
  vector(1,2) = y1;

%---
% second point
%---

  vector(2,1) = x2;
  vector(2,2) = y2;

%---
% third point
%---

  vector(3,1) = x2+( dxi*cs+dyi*sn)*tip;
  vector(3,2) = y2+(-dxi*sn+dyi*cs)*tip;

%---
% fourth point
%---

  vector(4,1) = x2;
  vector(4,2) = y2;

%---
% fifth point
%---

  vector(5,1) = x2+(dxi*cs-dyi*sn)*tip;
  vector(5,2) = y2+(dxi*sn+dyi*cs)*tip;

%-----
% done
%-----

  return