[R] Equal number of cuts in a contourplot with trellis

[Jesus Frias]

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ús María Frías 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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