[R] Date on x-axis of xyplot

[Deepayan Sarkar]

On Wednesday 17 September 2003 08:51, David James wrote:
> Prof Brian Ripley wrote:
> > On Tue, 16 Sep 2003, Deepayan Sarkar wrote:
> > > Is the date class standard enough to warrant including a check for it
> > > in lattice ?
> >
> > I don't think so.  The POSIX*t classes in R are the most standard,
> > followed by the chron package and only then the date package.
>
> If it is not overly complicated to implement, could I timidly suggest
> *not* checking for specific classes inside lattice, but rather use
> some other kind of mechanism (perhaps invoking helper functions,
> or use specific methods, etc.) to render axes?

This is definitely a good idea generally, and lattice tries to do that as much 
as possible (exceptions being splom, cloud, wireframe). However, at some 
point the tick locations and labels need to be calculated by some variation 
of pretty and format, which loses the class attribute and has to be processed 
case by case. 

In fact for POSIXct, pretty doesn't return sensible values, and that case has 
to be handled completely separately. The base plot functions seem to handle 
this via axis.POSIXct, which is not directly reusable and so is essentially 
repeated in lattice. It might be a good idea to eventually separate the 
common part out as something similar to pretty.

I also just dicovered that POSIXct handling in lattice is broken, except when 
scales$relation = "free" or "sliced". I'll fix this ASAP.

Deepayan