[R] format secondary axis for dates

[David Winsemius]

On Nov 16, 2010, at 2:32 PM, Jannis wrote:

> Dear List,
>
>
> this may be a Newbi question and may have been asked several times,  
> but i am too stupid to find the posts.
>
>
> I have a plot of values against POSIXct time steps. If I want to add  
> a second x axis to the top margin of the plot, only numbers are at  
> the tickmarks. Is there a straightforward way to specify the format  
> to convert them to character representations (similar to the labels  
> at the bottom)?
>
> x<-as.POSIXct(1:1000*(60^2),origin='01-01-1970')

The origin argument was incorrect and you got the wrong starting point:
 > x[1]
[1] "0001-01-19 01:00:00 EST"


> y=rnorm(1000)
> plot(x,y)
> axis(3)

This gives ticks at hourly intervals:

axis(3, labels=format(x, "%Y-%m-%d"), at=x)

Whereas this is probably what you want (after correcting the origin to  
"1970-01-01"

axis.POSIXct(3, x=x, labels=TRUE )

-- 
David.



>
>
> Thanks for your help
> Jannis
>
>
>
>
>
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David Winsemius, MD
West Hartford, CT