# [R] Problems plotting and regression w.r.t. date data type on x axis

PIKAL Petr petr.pikal at precheza.cz
Thu Feb 14 14:23:12 CET 2013

```Hi

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Kategoricus
> Sent: Thursday, February 14, 2013 12:25 PM
> To: r-help at r-project.org
> Subject: Re: [R] Problems plotting and regression w.r.t. date data type
> on x axis
>
> Hello Petr!
>
> thanks a lot for your help. The plot command plots well, but the abline
> returns without error and does nothing.
>
> > str(tabelle)
> 'data.frame':   4 obs. of  3 variables:
>  \$ date  : Factor w/ 4 levels "2009-01-1","2009-12-1",..: 1 2 3 4  \$
> number: int  1673 2111 2487 4301  \$ date2 : POSIXlt, format: "2009-01-
> 01" "2009-12-01" ...

As expected date is factor, number is numeric and date2 is POSIXlt format.

> > plot(tabelle\$date2,tabelle\$number, pch=19, xlab="date of retrieval",
> > ylab="number of animals") regression.punkte<-lm(tabelle\$number ~
> > as.Date(tabelle\$date)) regression.punkte

Here you are plotting number against POSIX date2.

>
> Call:
> lm(formula = tabelle\$number ~ as.Date(tabelle\$date))

Here you use date probably converted by as.Date. Why you do not use

lm(number ~ date2, data = tabelle)

which is usual form of model formula.

Try also change date2 by as.POSIXct.

Regards
Petr

>
> Coefficients:
>           (Intercept)  as.Date(tabelle\$date)
>            -24046.326                  1.799
>
> > abline(regression.punkte, lwd=2)
> >
>
>
>
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