  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, yc, 0.75, 'r' )
  grid on
  axis ( [0, 2, 0, 5.0 ] )
  plot ( x, y, 'k', 'linewidth', 3 )
  hold off

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

