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" ...

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

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

Coefficients:
          (Intercept)  as.Date(tabelle$date)  
           -24046.326                  1.799

abline(regression.punkte, lwd=2)

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