Label lines in a plot

Question

I am plotting two lines using

plot(x, y, type = \"l\", color = \"red\")

and

points(x2, y2, type = \"l\", color = \"blue\")         


        

Answer 1

locator() is an interactive method of obtaining coordinates by clicking on an existing graph.

Here are instructions on how to use locator() to find the right coordinates for a label on a graph.

Step 1: Plot a graph:

plot(1:100)

Step 2: Type the following into the console:

coords <- locator()

Step 3: Click once on the plot, then click Stop .. Stop Locator at the top left of the plot (this returns control back to the R console).

Step 4: Find the returned coordinates:

coords
$x
[1] 30.26407
$y
[1] 81.66773

Step 5: Now, you can add a label to the existing plot using these coordinates:

text(x=30.26407, y=81.66773,label="This label appears where I clicked")

or

text(x=coords$x, y=coords$y,label="This label appears where I clicked")

Here is the result:

You'll notice that the label appears with its center where you clicked. Its better if the label appears with its first character where you clicked. To find the correct parameter, see the help for text, and add the parameter pos=4:

text(x=30,y=80,pos=4,label = "hello")

Notes:

• The label appears in the same x,y coordinates as dots on the graph. So, x=100,y=0 would appear on the lower right, while x=0,y=100 would appear on the upper left.
• Can also use legend() to plot a label (this draws a box around the label which often looks nicer).
• See How to change font family in a legend in an R-plot? for how to change the font in a legend, and how to auto-place the legend on the top right of the graph.
• I would recommend becoming familiar with ggplot2 instead of plot, as ggplot2 is the gold standard for producing graphs.

Answer 2

You can use the locator() within text() by point&click method.

y <- rnorm(100, 10)
y2 <- rnorm(100, 20)
x <- 1:100

plot(x, y, type = "n", ylim = c(0, 40), xlim = c(0, 120))
lines(x, y)
lines(x, y2, col = "red")
text(locator(), labels = c("red line", "black line)"))

Answer 3

To use directlabels, you must structure your data in a data.frame and then use a high-level plotting system like ggplot2, or in the example below, lattice:

y <- rnorm(100, 10)
y2 <- rnorm(100, 20)
x <- 1:100
treatment <- rep(c("one group","another"),each=length(x))
df <- data.frame(x=c(x,x),y=c(y,y2),treatment)
library(lattice)
p <- xyplot(y~x,df,groups=treatment,type="l")
if(!require(directlabels)){
  install.packages("directlabels")
  library(directlabels)
}
print(direct.label(p))
print(direct.label(update(p,xlim=c(0,120)),last.points))

Answer 4

Instead of using locator(), you could also just make the label coordinates a function of your data. For example, piggy backing on Roman's demo:

text(x=rep(max(x)+3, 2), y=c(mean(y), mean(y2)), pos=4, labels=c('black line', 'red line'))