This is the plot method for 
  quadrature schemes (objects of class "quad",
  see quad.object).
First the data points of the quadrature scheme
  are plotted (in their observation window) using 
  plot.ppp with any arguments specified in ...
Then the dummy points of the quadrature scheme are plotted
  using plot.ppp with any arguments specified in
  dum.
By default the dummy points are superimposed onto the plot of data
  points. This can be overridden by including the argument
  add=FALSE in the list dum as shown in the examples.
  In this case the data and dummy point patterns are plotted separately.
See par and plot.ppp
  for other possible arguments controlling the plots.