These functions can serve as one of the arguments panel,
  lower.panel, upper.panel, diag.panel
  passed to graphics commands like
  pairs or coplot,
  to determine what kind of plotting is done in each panel
  of a multi-panel graphical display. In particular they work
  with pairs.im.
The functions panel.contour and panel.contour
  are suitable for the off-diagonal plots which involve
  two datasets x and y.
  They first rescale x and y to the unit square,
  then apply kernel smoothing with bandwidth sigma
  using density.ppp.
  Then panel.contour draws a contour plot
  while panel.image draws a colour image.
The function panel.histogram is suitable for the
  diagonal plots which involve a single dataset x.
  It displays a histogram of the data.