an optional list used by selected plot functions to create
custom axes. If this axes argument is not
NULL, functions plot.basisfd, plot.fd,
plot.fdSmooth plotfit.fd, plotfit.fdSmooth, and
plot.Lfd will create axes via x$axes[[1]] and
x$axes[-1]. The primary example of this uses
list("axesIntervals", ...), e.g., with Fourier bases
to create CanadianWeather plots