date in plot, can't add regression line

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
project.org] On Behalf Of Norbert Skalski
Sent: Tuesday, August 28, 2012 9:49 AM
To: r-help at r-project.org
Subject: [R] date in plot, can't add regression line

Hello all,

I have been using R for about 3 weeks and I am frustrated by a problem.
I have read R in a nutshell, scoured the internet for help but I either
am not understanding examples or am missing something completely basic.
Here is the problem:

I want to plot data that contains dates on the x axis.  Then I want to
fit a line to the data.  I have been unable to do it.

This is an example of the data (in a dataframe called
"tradeflavorbyday"), 40 lines of it (I'm sorry it's not in a runnable
form, not sure how to get that from R) :
     tradeflavor  timestamp   x
1              1 2009-01-22   1
2              2 2009-01-22   1
3              1 2009-01-23   1
4              1 2009-01-27  54
5              1 2009-01-28 105
6              2 2009-01-28   2
7             16 2009-01-28   2
8              1 2009-01-29  71
9             16 2009-01-29   2
10             1 2009-01-30  42
11             1 2009-02-02  19
12            16 2009-02-02   2
13             1 2009-02-03  36
14             4 2009-02-03   2
15             8 2009-02-03   3
16             1 2009-02-04  73
17             8 2009-02-04  12
18            16 2009-02-04   7
19             1 2009-02-05  53
20             8 2009-02-05   6
21            16 2009-02-05   9
22             1 2009-02-06  38
23             4 2009-02-06   6
24             8 2009-02-06   2
25            16 2009-02-06   3
26             1 2009-02-09  42
27             2 2009-02-09   2
28             4 2009-02-09   1
29             8 2009-02-09   2
30             1 2009-02-10  87
31             4 2009-02-10   2
32             8 2009-02-10   4
33            16 2009-02-10   3
34             1 2009-02-11  55
35             2 2009-02-11   6
36             4 2009-02-11   4
37             8 2009-02-11   2
38            16 2009-02-11   8
39             1 2009-02-12 153
40             2 2009-02-12   6


The plot displays the x column as the yaxis and the date as the x axis,
grouped by the tradetype column.
The timestamp column:

class(tradeflavorbyday$timestamp)

[1] "POSIXlt" "POSIXt"

So in this case I want to plot tradetype 1 (method 1):

xdates <- tradeflavorbyday$timestamp[tradeflavorbyday$tradeflavor == 1]
ydata <- tradeflavorbyday$x[tradeflavorbyday$tradeflavor == 1]

plot(xdates, ydata, col="black", xlab="Dates", ylab="Count")

Up to here it works great.

Now a abline through lm:

xylm <- lm(ydata~xdates)   <------ this fails, can't do dates as below
abline(xylm, col="black")

lm(ydata~xdates)

Error in model.frame.default(formula = ydata ~ xdates,
drop.unused.levels = TRUE) :
  invalid type (list) for variable 'xdates'