R help - May 2011 - Using dates on axis with Grid plots

Hi,

I'm trying to use Grid plots and would like to have an X axis that
represents dates. I have several years of data so I would like to be able to
have labeled tick marks only intermittently (not one per date).  I can
transform the initial data from a date time string into POSIXlt or POSIXct,
or Date objects.

The issue is that when I try to layout the plot using:

dev.off()
  pushViewport(
    plotViewport(
      c(5, 5, 4, 2),
        xscale=c(
           min(foo),
           max(foo)),
           yscale=c(0,30)
    )
)

I get this error, regardless of whether "foo" is formated as POSIXlt,
POSIXct or Date for the xscale.

Error in valid.viewport(x, y, width, height, just, gp, clip, xscale, yscale,
 :
  Invalid 'xscale' in viewport

How does one specify the scale?  In the end, I would prefer something like
"2007   2008", etc. to appear on the axis and, of course, I'd
actually want
to plot the data with an x axis of dates. I'm not yet far enough to see if
that's going to be an issue.


Apologies if this is a stupid question, but it's very hard to search for
Grid examples on the web, because "grid" is a very common term in R
plotting
generally.

	[[alternative HTML version deleted]]

Hi

On 15/05/2011 2:01 a.m., Larry White wrote:
> Hi,
>
> I'm trying to use Grid plots and would like to have an X axis that
> represents dates. I have several years of data so I would like to be able
to
> have labeled tick marks only intermittently (not one per date).  I can
> transform the initial data from a date time string into POSIXlt or POSIXct,
> or Date objects.
>
> The issue is that when I try to layout the plot using:
>
> dev.off()
>    pushViewport(
>      plotViewport(
>        c(5, 5, 4, 2),
>          xscale=c(
>             min(foo),
>             max(foo)),
>             yscale=c(0,30)
>      )
> )
>
> I get this error, regardless of whether "foo" is formated as
POSIXlt,
> POSIXct or Date for the xscale.
>
> Error in valid.viewport(x, y, width, height, just, gp, clip, xscale,
yscale,
>   :
>    Invalid 'xscale' in viewport
>
> How does one specify the scale?  In the end, I would prefer something like
> "2007   2008", etc. to appear on the axis and, of course, I'd
actually want
> to plot the data with an x axis of dates. I'm not yet far enough to see
if
> that's going to be an issue.
The viewport scale has to be numeric.  Dates are not supported at all by 
grid.xaxis() or grid.yaxis().  If your dates are just days, you can 
as.numeric() them to get a useful range and then pretty() and 
as.Date(origin="1970-01-01") to get useful labels for tick marks, but 
the general solution is more complex (see axis.Date()).  Depending on 
what your plots need to look like, another approach would be to 
customize a 'lattice' plot because it provides support for date-based 
axes.

Paul
>
> Apologies if this is a stupid question, but it's very hard to search
for
> Grid examples on the web, because "grid" is a very common term in
R plotting
> generally.
>
> 	[[alternative HTML version deleted]]
>
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-- 
Dr Paul Murrell
Department of Statistics
The University of Auckland
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paul at stat.auckland.ac.nz
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