R-beta: image(x,y,z)?
Thomas Lumley
thomas at biostat.washington.edu
Tue Apr 28 17:59:01 CEST 1998
On Tue, 28 Apr 1998, Bill Simpson wrote:
>
> I can get at data$x, data$y, data$z. I want to do an image plot. Ideally
> image (or a relative of image) would accept the vectors data$x, data$y,
> data$z as arguments. (After all, if you can do plot(x,y) on vectors x and
> y, why can't you do image(x,y,z) on vectors x, y, and z?) However it
> doesn't work like that.
>
The real reason is that it isn't written that way. There is a good
rationalisation, though. image() is a surface plot, not a 3-d
scatterplot, and so it is necessary that z is defined at every point on
the x,y grid (otherwise we don't know what color to plot). This is hard to
ensure when you have image(x,y,z) for vectors
> Anyway, can someone please explain how to get my data into a form
> acceptable to image? Thanks very much!
In fact, you don't need z to be a matrix. What you need is
length(z)=length(x)*length(y)
x increasing
y increasing
The idea is that values of z are ordered in the same way as if it were
generated by outer(x,y,somefunction), which will generally be how it was
generated.
We probably shouldn't require x and y to be increasing -- we could still
assume z is ordered as if it were outer(x,y,somefunction).
Anyway, one thing you can do if you have the full x and y vectors is
image(unique(x),unique(y),z)
If you have full-length x and y vectors that are not in the right order
you can use
o<-order(y,x)
image(unique(x[o]),unique(y[o]),z[o])
which should probably be built in to image().
Thomas Lumley
------------------------
Biostatistics
Uni of Washington
Box 357232
Seattle WA 98195-7232
------------------------
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