[R] xyplot() with discontinuous x-axis variable

[Rich Shepard]

   My xyplot() with superposed multiple condiions looks better with lines
than with points (it's easier to see changes over time with the lines). But,
there are gaps in the years (the x axis) for which there are data to be
plotted. For example, there are data for years 2004-2006 and 2010-2012, but
not for 2007-2009. I would like to have the lines for only the two groups
with data.

   Reading ?xyplot suggests that the group attribute might do the job but I
do not see how to write the equation.

   Is it possible to plot with lines on discontinuous data or should I use
large, solid circles for each year's data points?

Rich