  a = 0.0;
  b = 2.0;
  n = 21;
  x = linspace ( a, b, n );
  y = 2 * ( x + 0.25 ) .* ( 2.75 - x );
  xc(1:n-1) = 0.5 * ( x(1:n-1) + x(2:n) );
  yc = 2 * ( xc + 0.25 ) .* ( 2.75 - xc );
  clf
  hold on
  bar ( xc(1:4:20), yc(1:4:20), 0.20, 'r' )
  bar ( xc(2:4:20), yc(2:4:20), 0.20, 'g' )
  bar ( xc(3:4:20), yc(3:4:20), 0.20, 'b' )
  bar ( xc(4:4:20), yc(4:4:20), 0.20, 'm' )
  grid on
  axis ( [0, 2, 0, 5.0 ] )
  plot ( x, y, 'k', 'linewidth', 3 )
  hold off

  print ( '-dpng', 'quad_picture_parallel2.png' )

