ggplot中日期轴的中点

Question

I want to add some labels to a plot on which the x-axis is a Date. I want the label to be centered around the middle. How do I find the midpoint on the x-axis?

Example:

example <- data.frame(time = c("02/26/11", "05/26/10", "05/27/10", "05/28/10",   
                               "05/29/10", "06/27/10", "06/30/10", "10/27/10", 
                               "10/27/10", "12/26/12"),
                      value = c(5, 1, 7, 8, 11, 20, 14, 1, 20, 12))

example$time <- as.Date(example$time, format = "%m/%d/%Y")

ggplot(example, aes(x = time, y = value)) + geom_point() + 
    scale_x_date(labels = date_format("%b%Y"),
                 breaks = "3 month",
                 minor_breaks = "1 month")

Now, I want to use geom_text to add a text lable that has x coordinate positioned int he middle of the x-axis, and y at the middle of the y-axis.

Answer 1

It sounds like you just want something like

xx<-data.frame(
    time=mean(range(example$time)),
    value=mean(range(example$value))
)

ggplot(example, aes(x = time, y = value)) + geom_point() + 
    geom_text(data=xx, label="midtext") +
    scale_x_date(labels = date_format("%b%Y"),
                 breaks = "3 month",
                 minor_breaks = "1 month")

We find the center of the plot by finding the center of the ranges of each of the axes. Then we use theses values in the call to geom_text .

That will produce this picture.

PS. I also changed your date formatting line to

example$time <- as.Date(example$time, format = "%m/%d/%y")

since you only have two-digit years and not 4 digit years.