[R] image function with date-time on X axis

Peter Ehlers ehlers at ucalgary.ca
Fri Apr 30 04:05:36 CEST 2010

On 2010-04-29 12:15, Halldór Björnsson wrote:
> I am trying to plot a image where the x axis has the units of time.
> When I issue the
> image(x,y,z) command with x as a POSIXct object, it fails to put a
> time stamp on the
> x axis.
> Instead I get a warning "Incompatible methods" warning and no dates on
> my x axis.
> This example shows my problem:
> Rmat=t(matrix(data=rnorm(1:500),ncol=10,nrow=50))
> tax=seq(ISOdate(2010,4,14,12,0,0), ISOdate(2010,4,19,00,0,0), by = "12
> hours") ylevs=seq(100,5000,length=50)
> image(tax,ylevs,Rmat)
> This givies the warning:
> In image.default(tax, zlevs, Rmat) :
>    Incompatible methods ("-.POSIXt", "Ops.difftime") for "-"
> 2: In is.vector(X) :

I don't get that warning. Are you doing something you're not
telling us? (I note that you have 'ylevs' in one place and 'zlevs'
in another).

This works for me (using your above definitions of Rmat, etc):

  image(tax, ylevs, Rmat, xaxt = "n")
  axis(1, at = as.numeric(tax), lab = weekdays(tax, TRUE))

  -Peter Ehlers

> Contrast the x axis result of image with that of plot
>   plot(tax,rnorm(length(tax)))
> where the date-time shows stamps work fine.
> How can I get image to behave (or is there a better function for the job?)
> Sincerely
> Halldór
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Peter Ehlers
University of Calgary

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