Plot SIR spline lines with R base graphics
# S3 method for sirspline
lines(x, conf.int = TRUE, print.levels = NA,
  select.spline, ...)an object returned by function sirspline
logical; default TRUE draws also the 95 confidence intervals
name(s) to be plotted. Default plots all levels.
select which spline variable (a number or a name) is plotted.
arguments passed on to lines()
In lines.sirspline most of graphical parameters is user 
adjustable.
Desired spline variable can be selected with select.spline and only one
can be plotted at a time. The spline variable can include 
several levels, e.g. gender (these are the levels of print
from sirspline). All levels are printed by default, but a
specific level can be selected using argument
print.levels. Printing the levels separately enables  e.g. to
give different colours for each level.
Other sir functions: plot.sirspline,
  sir_exp, sir_ratio,
  sirspline, sir