[R] Fwd: lattice, panel.grid, and scales=list(tick.number=XXX)

[Deepayan Sarkar]

I just realised that I had forgotten to copy this to the list.

---------- Forwarded message ----------
From: Deepayan Sarkar <deepayan.sarkar at gmail.com>
Date: Jun 15, 2005 12:13 PM
Subject: Re: [R] lattice, panel.grid, and scales=list(tick.number=XXX)
To: Berton Gunter <gunter.berton at gene.com>


On 6/14/05, Berton Gunter <gunter.berton at gene.com> wrote:
> If you look at the code of panel.grid, you'll see why  it doesn't work -- it
> does not use any of the scale parameters. Moreover the Help page for
> panel.grid explicitly warns that the h,v=-1 specification may not work, so
> no promises have been broken.


Right, the determination of tick mark locations can be arbitrarily
complicated, via the use of methods of the (unexported) generic
'formattedTicksAndLabels'.  The only way I can think of to make
panel.grid aware of that is to store the results of that computation
somewhere. Note that these results could potentially be different for
every panel. While this is not impossible, it violates the principle
that a panel function only has access to the data for that panel (not
that this isn't ever violated, but not to this extent), and I would be
inclined to keep things as they are unless there are better reasons.

In this particular case, the tick computations are simple, so one
could do (using the recently added 'current.panel.limits', only
available if you update lattice to the latest version):

bwplot(voice.part ~ height, data=singer,
       scales = list(tick.number = 10),
       panel = function(...) {
           ref.line <- trellis.par.get("reference.line")
           panel.abline(v = pretty(current.panel.limits()$xlim, n = 10),
                        col = ref.line$col,
                        lty = ref.line$lty,
                        lwd = ref.line$lwd)
           panel.bwplot(...)
       })

[...]
> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of M. K.
> Sent: Tuesday, June 14, 2005 3:37 PM
> To: R-help mailing list
> Subject: [R] lattice, panel.grid, and scales=list(tick.number=XXX)
>
> I have a Lattice plot in which I want to adjust the number of tick
> marks used, and I want to have the drawn grid reflect that change.
> Here is what I'm doing:
>
> bwplot(var1 ~ var2, data=df, scales=list(tick.number=10),
>        panel=function(...) {
>            panel.grid(h=0,v=-1,...);
>            panel.stripplot(col="gray40", pch="|", cex=2, ...);
>            panel.bwplot(...);
>            })
>
> Unfortunately this doesn't quite work.  Although the bwplot's tick
> marks are indeed increased as requested, the panel.grid produces the
> same (3 line) grid as before, seemingly unaware of the changed # of
> ticks.