R X-axis Date Labels using plot()

Question

Using the plot() function in R, I'm trying to produce a scatterplot of points of the form (SaleDate,SalePrice) = (saldt,sapPr) from a time-series, cross-section real estate sales dataset in dataframe format. My problem concerns labels for the X-axis. Just about any series of annual labels would be adequate, e.g. 1999,2000,...,2013 or 1999-01-01,...,2013-01-01. What I'm getting now, a single label, 2000, at what appears to be the proper location won't work.

The following is my call to plot():

plot(r12rgr0$saldt, r12rgr0$salpr/1000, type="p", pch=20, col="blue", cex.axis=.75, 
     xlim=c(as.Date("1999-01-01"),as.Date("2014-01-01")),
     ylim=c(100,650), 
     main="Heritage Square Sales Prices $000s 1990-2014",xlab="Sale Date",ylab="$000s")

The xlim and ylim are called out to bound the date and price ranges of the data to be plotted; note prices are plotted as $000s. r12rgr0$saldt really is a date; str(r12rgr0$saldt) returns:

Date[1:4190], format: "1999-10-26" "2013-07-06" "2003-08-25" NA NA "2000-05-24"  xx 

I have reviewed several threads here concerning similar questions, and see that the solution probably lies with turning off the default X-axis behavior and using axis.date, but i) At my current level of R skill, I'm not sure I'd be able to solve the problem, and ii) I wonder why the plotting defaults are producing these rather puzzling (to me, at least) results?

Addl Observations: The Y-axis labels are just fine 100, 200,..., 600. The general appearance of the scatterplot indicates the called-for date ranges are being observed and the relative positions of the plotted points are correct. Replacing xlim=... as above with xlim=c("1999-01-01","2014-01-01")

or

xlim=c(as.numeric(as.character("1999-01-01")),as.numeric(as.character("2014-01-01")))

or

xlim=c(as.POSIXct("1999-01-01", format="%Y-%m-%d"),as.POSIXct("2014-01-01", format="%Y-%m-%d"))

all result in error messages.

Solution

With plots it's very hard to reproduce results with out sample data. Here's a sample I'll use

dd<-data.frame(
  saldt=seq(as.Date("1999-01-01"), as.Date("2014-01-10"), by="6 mon"),
  salpr = cumsum(rnorm(31))
)

A simple plot with

with(dd, plot(saldt, salpr))

produces a few year marks

If i wanted more control, I could use axis.Date as you alluded to

with(dd, plot(saldt, salpr, xaxt="n"))
axis.Date(1, at=seq(min(dd$saldt), max(dd$saldt), by="30 mon"), format="%m-%Y")

which gives

note that xlim will only zoom in parts of the plot. It is not directly connected to the axis labels but the axis labels will adjust to provide a "pretty" range to cover the data that is plotted. Doing just

xlim=c(as.Date("1999-01-01"),as.Date("2014-01-01"))

is the correct way to zoom the plot. No need for conversion to numeric or POSIXct.

Comments

[jackw19]

Thanks very much for your helpful comment.

[jackw19]

I think maybe plot() draws a chart, including the pretty axes labels, before it zooms the view a la xlim or ylim. I have a couple sales date outliers in my dataframe, including an obs in 1909. When I plot() without using xlim, that is including the outlier, I get a nice set of X-axis labels at 20 yr intervals starting at 1920; all but one of the graphed points are squeeaed way over on the rhs of the chart. And the only label under the big concentration of points is 2000 -- right in the same relative position in which it appears in the zoomed version using xlim. Can't be a coincidence.