R help - Jan 2006 - Equal number of cuts in a contourplot with trellis

Dear R-helpers,

	I need some help to produce a set of contour plots that I am
trying to make in order to compare surfaces between the levels of a
factor. For example: 

library(lattice)
g <- expand.grid(x = 60:100, y = 1:25, ti =
c("a","b","c"))
g$z <-with(g,
  (-1e-4*x-1e-3*y-1e-5*x*y)*(ti=="a") +
  (1e-2*x-1e-3*y-1e-4*x*y)*(ti=="b") +
  (1e-3*x-1e-3*y-1.e-5*x*y)*(ti=="c")
               )

contourplot(z~ x * y|ti, data = g,
            cuts=20,
            pretty=T,
            screen = list(z = 30, x = -60))

As you can see in the figure, most of the contour lines are in one of
the levels and we are not able to see how the other levels look like.

I would like to display the same number of cuts in each of the trellis.
I can make each of the contourplots separately and control the number of
cuts but I am not able to plot all of them in one. 

Thanks in advance,

Jesus

Jes??s Mar??a Fr??as Celayeta
School of Food Science and Environmental Health
Dublin Institute of Technology
Cathal Brugha St. Dublin 1
p + 353 1 4024459
f + 353 1 4024495
w http://www.dit.ie/DIT/tourismfood/science/



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On 1/12/06, Jesus Frias <Jesus.Frias at dit.ie>
wrote:
> Dear R-helpers,
>
> 	I need some help to produce a set of contour plots that I am
> trying to make in order to compare surfaces between the levels of a
> factor. For example:
>
> library(lattice)
> g <- expand.grid(x = 60:100, y = 1:25, ti =
c("a","b","c"))
> g$z <-with(g,
>   (-1e-4*x-1e-3*y-1e-5*x*y)*(ti=="a") +
>   (1e-2*x-1e-3*y-1e-4*x*y)*(ti=="b") +
>   (1e-3*x-1e-3*y-1.e-5*x*y)*(ti=="c")
>                )
>
> contourplot(z~ x * y|ti, data = g,
>             cuts=20,
>             pretty=T,
>             screen = list(z = 30, x = -60))
>
> As you can see in the figure, most of the contour lines are in one of
> the levels and we are not able to see how the other levels look like.
>
> I would like to display the same number of cuts in each of the trellis.
> I can make each of the contourplots separately and control the number of
> cuts but I am not able to plot all of them in one.
The simplest solution is to recompute the levels for each panel function:

contourplot(z~ x * y|ti, data = g,
            label.style = "align",
            panel = function(x, y, z, subscripts, at, ...) {
                at <- pretty(z[subscripts], 10)
                panel.contourplot(x, y, z,
                                  subscripts = subscripts,
                                  at = at,
                                  ...)
            })

Alternatively, you could pass in a suitable 'at' vector computed
externally.

Deepayan
--
http://www.stat.wisc.edu/~deepayan/