Question

Is it possible to use R to add the date onto a ggplot graph (upon which the graph data is based)? I have code which selects the most daily data over a the course of a week, and this subsequently gets plotted. An example dataframe (from a previous answer) is shown below:

set.seed(1234)
df <- data.frame(Date=seq(as.POSIXct("05:00", format="%H:%M"), 
                          as.POSIXct("23:00", format="%H:%M"), by="hours"))
df$Counts <- sample(19)
df <- df[-c(4,7,17,18),]

# generate the groups automatically and plot
idx <- c(1, diff(df$Date))
i2 <- c(1,which(idx != 1), nrow(df)+1)
df$grp <- rep(1:length(diff(i2)), diff(i2))
g <- ggplot(df, aes(x=Date, y=Counts)) + geom_line(aes(group = grp)) + 
  geom_point()
g

I know I can use the following code to get R to work out which day of the week my data is:

d <- as.Date("19/02/2013","%d/%m/%Y") 
format(d, "%A, %b %d, %Y")

But I obviously have to manually input the date for this to work.

Is it possible to get R to read the dataframe and work out the day of the week the data is based on (it will be only one day), and then add this as a legend in the top right hand corner of the graph? Ideally the format would include both the day and date (e.g. Tuesday, Feb 19th 2013).

Edit

I want to add the date as text in the graph. Can I use geom_text for this? This isn’t working at the moment, but so far I have got:

g + geom_text(aes(x=as.POSIXct("18:00:00", format="%H:%M:%S"), y=1500), label=date_format("%A, %b %d, %Y")", size=2)

Can anyone advise me what I am doing wrong?

Answer 1

As suggested by Arun(see comment) you can you can format your input data or add a formatting to the current plot :

 library(scales)
 g+ scale_x_datetime(labels = date_format("%A, %b %d, %Y"))

EDIT

I guess you want to annotate your graph adding a date indicator

Since you will add a single date , you can use annotate, It is better than using geom_text which will overlap many dates. For example:

 label <- unique(format(as.Date(df$Date, "%d/%m/%Y"),
                                "%A, %b %d, %Y"))
 g+ annotate("text", x = mean(df$Date), y = 20, 
       label = label,size=20, col='red')