R: Finding the location of tick marks when plotting dates - r

Question

I'm running an R script which plots data over time using plot and some data taken as.Date. I am then adding using grid to add guidelines like so:
plot(as.Date(data[["Date"]]), -data[["dbns"]], lwd=1, col = colours[i], type="l",ylim=ylimit, xlim=xlimit, xlab="", ylab =ylabel)
grid(NULL,NULL,lwd=1)
Below is an example of how this comes out (actually example is after this plot is looped over 3 times to add each line)
The problem I am having is that the guidelines added with grid do not line up with the default tick marks added by plot. The plot function by default adds tick every 10 years, which is what I want. Looking at axTicks(1) gives me 0 5000 10000 15000 which is the locations of guidelines in units of days after 1/1/1970.
Is there a way to find out the location of the tickmarks that have been added? And more importantly, is there then a way to set the guidelines to match these locations?
I realise that for this example the ticks are [0, 365.25*10, 365.25*20 etc.] but an answer which solves a general case would be nice.
Thanks
JP
EDIT:
Here is a reproducible example to show exactly what I mean.
data <- read.csv("test.csv"), header =TRUE)
data[["Date"]] <- as.Date(data[["Date"]], format = "%d/%m/%Y")
data<-data[order(as.Date(data$Date)),]
plot(as.Date(data[["Date"]]), -data[["dbns"]], type="l", xlim=xlimit)
grid(NULL,NULL,lwd=3)
Where the file test.csv looks like this (extract of actual data)
bore Date dbns
42230210A 20/01/2009 13.13
42230210A 8/05/2009 13.21
42230210A 18/06/2009 13.19
42230210A 3/08/2009 13.2
42230210A 2/09/2009 13.25
42230210A 1/10/2009 13.3
42230210A 3/12/2009 13.32
42230210A 24/02/2010 13.3
42230210A 18/05/2010 13.3
42230210A 25/04/2012 11.45
42230210A 27/09/1966 11.18
42230210A 28/10/1969 11.14
42230210A 6/01/1970 11.03
42230210A 5/06/1973 10.68
42230210A 28/08/1973 10.63
42230210A 2/10/1973 10.73
42230210A 4/12/1973 10.7
42230210A 20/02/1980 11.39
42230210A 29/04/1980 11.45
42230210A 27/08/1980 11.45
42230210A 22/06/1988 11.14
42230210A 27/02/1996 11.78
42230210A 6/08/1996 11.68
42230210A 8/02/2000 11.64
42230210A 8/06/2000 11.71
42230210A 7/09/2000 11.75
42230210A 15/07/2008 13.01
The output of which is below. The guidelines are offset from the tick marks?

Answer 1

So checking ?grid suggests that what's going on is that it is placing the grid lines based on the "default" tick mark locations, but since you plotted a date object, R didn't use the "default" (in the sense of treating the date as a numeric variable) to place the tick marks.
The documentation further notes that if you need finer control, you should use abline to place them directly.
So you probably want to do this:
plot(as.Date(data[["Date"]]), -data[["dbns"]], type="l")
grid(NA,NULL,lwd=3)
v <- as.numeric(as.Date(paste0(c(1970,1980,1990,2000,2010),'-01-01')))
abline(v = v,lty = "dotted",col = "lightgray",lwd = 3)

Answer 2

There is a function Axis.Date which creates appropriately pretty axis tick locations for an axis for a Date object. This does not change par('xaxp') which grid uses to find the locations of the tick marks.
We can get around by creating the axis manually (and saving the values)
# assuming your data is called DD
DD$Date <- as.Date(as.character(DD$Date), format = '%d/%m/%Y')
# don't plot the x axis (xaxt = 'n')
plot(-dbns~Date,data = DD[order(DD$Date),], type="l", xaxt = 'n')
# create the axis and save the tick locations
at <- as.numeric(Axis(side = 1, x = DD$Date))
grid(NA,NULL,lwd=3)
abline(v = at,lty = "dotted",col = "lightgray",lwd = 3)

Answer 3

The answer to your questions are the following ones:
Is there a way to find out the location of the tickmarks that have been added?
Just after the plot function call, use the function axis.Date
x_ticks <- axis.Date(1, data$Date, labels = FALSE)
Variable x_ticks contains the location of the ticks used in the x axis (identified by 1 in the first parameter of the function).
And more importantly, is there then a way to set the guidelines to match these locations?
grid(NA, NULL, lwd=3)
x_ticks <- axis.Date(1, data$Date, labels = FALSE)
abline(v = x_ticks, lty = 3, col ="lightgray", lwd = 3)
With the NA in the first parameter of the grid function (nx = NA), you avoid that the vertical guidelines are printed in the wrong places. The second gives you the location of the ticks. Finally, the third prints the guidelines in the right locations.
In this way, the plot you obtain looks like, I guess, the one you expected