Strange abline behaviour

Question

I am currently trying to graph a time series of a price spread and then add an abline with a regression. Currently this is just a AR(1) because I wanted to get the plot right before starting.

The data is from a .XLS and is organized as such in OpenOffice

Date - Price1 - Price 2 - NA.(Empty) 
01.01.1982 - 1.56 - 2.53 -  
[...]

I am reading this as

library(xlsx)
library(AER)
x<-read.xlsx("data.xlsx",1)

then I fill the empty column like so

x$NA.=(x$Price1-x$Price2)

So I now have a table in memory that produces this head()

       Date Price1 Price2 NA.
1 1987-08-28  18.30  19.44 1.24
2 1987-08-31  18.65  19.75 1.12

(there is an index column before Date, I don't really need it since I plot by date but its there)

I then do

plot(x$Date,x$NA.)

I get the correct plot. I have modified that plot command a bit to get correct grids, axis and dates and lines etc, but the problem I have also persists even with the simple plot version above, so the problem is not with my edits.

The problem is the following:

If I now try to plot a line

abline(a=1,b=1,col="blue")

It doesn't work. The command goes through, but it does not show a line. However:

abline(a=1,b=0,col="blue")

works as intended and shows a blue, horizontal line.

The problem I have is that I want to feed a regression object into the plot, for example like so

SPRC=zoo(x$NA.,x$Date)
SPRC2=lag(SPRC, -1)
SPRC=SPRC[2:length(SPRC)]
LMO<-lm(SPRC ~ SPRC2)
abline(a=LMO$coefficients[2],b=LMO$coefficients[1],col="red")

What I am trying to do is a simple AR to test things out. The regression works as intended, but the abline does not produce an output.

I also tried to do abline without variables - it works only if it b=0. I also tried instead to do a

plot(SPRC)

and then any kind of abline, but either none shows up or it becomes a vertical line (!). Only if b=0 it becomes a horizontal line.

I imagine this has to do with the data object or input, but I am really lost on why it does not work. I also tried an as.Date on the Date object, but that does not change anything. All other plot commands seem to work, like adding custom grids, par, locator texts, axis etc. The problem occurs if I start a clean R session and pretty much only enter the code above. I also tried to switch around the regression variables, the a and b values, or the order of the plot variables. It still does not work

Can you imagine what might be the issue?

Edit: I just checked the datatypes if typeof(). typeof x is "list", everything else is "double", even if I do a x$Date<-as.Date(x$Date,"%d.%m.%Y")

Edit2: I went ahead and saved the file as csv and read it with read.csv then I did

plot(x$Price1)

and

abline(a=40,b=1)

All it does is produce a vertical(!) line that is slightly turned clockwise. Is my R broken?

(I realize the scale is off for the price - the spread is around 0. But even with an a=40, the line is identical)

Answer 1

x<- read.table(text="Date Price1 Price2 NA.
 28.08.1987  18.30  19.44 1.24
 31.08.1987  18.65  19.75 1.12", sep="", header=TRUE)
x$Date <- as.Date(x$Date, "%d.%m.%Y")

plot(x$Date, x$NA.)

My locale settings are in french, so the x labels stands for Friday to Monday.

# If we're trying to find the actual coordinates of some points on the plot
# here is what we find:
locator() 
$x
[1] 6449.495 6448.035 6450.967

$y
[1] 1.182379 1.186610 1.182908

# The x axis is running from 6448 to 6451 and here is the reason:

x$Date # Here is your date vector
[1] "1987-08-28" "1987-08-31"
as.numeric(x$Date) # And here it is converted in numerics
[1] 6448 6451 # Hence the values found on the plot with locator.

# The default origin for dates is the first of January, 1970
# 6448 is the number of days from that date to the 28th of August 1987.

# But you can still use regression tools:
lm(NA.~Date, data=x)->lmx
abline(lmx)  # YOu don't actually need to break your lm object into its coefficients, abline recognize lm objects.

# And the same works with package zoo
library(zoo)
x<- read.table(text="Date Price1 Price2 NA.
 28.08.1987  18.30  19.44 1.24
 31.08.1987  18.65  19.77 1.12
 01.09.1987  18.65  19.75 1.10", sep="", header=TRUE)
x$Date <- as.Date(x$Date, "%d.%m.%Y")
SPRC<-zoo(x$NA.,x$Date)
SPRC2<-lag(SPRC, -1)
SPRC<-SPRC[2:length(SPRC)]
LMO<-lm(SPRC ~ SPRC2)
plot(SPRC)
abline(LMO)