R Tutorial Series: Labeling Data Points on a Plot

September 19, 2010
By

(This article was first published on R Tutorial Series, and kindly contributed to R-bloggers)

There are times that labeling a plot’s data points can be very useful, such as when conveying information in certain visuals or looking for patterns in our data. Fortunately, labeling the individual data points on a plot is a relatively simple process in R. In this tutorial, we will use the Calibrate package’s textxy function to label the points on a scatterplot.

Tutorial Files

Before we begin, you may want to download the sample data (.csv) used in this tutorial. Be sure to right-click and save the file to your R working directory. This dataset contains information used to estimate undergraduate enrollment at the University of New Mexico (Office of Institutional Research, 1990). Note that this tutorial assumes that this data has already been read into R and saved into a variable named enrollmentData.

Plot

To begin, we need to create a scatterplot using the plot(x,y) function. With our example data, we will plot the year on the x axis and the unemployment rate on the y axis.

1. > #generate a plot using the plot(x,y) function
2. > #plot year on the x axis and unemployment rate on the y axis
3. > plot(enrollmentData\$YEAR, enrollmentData\$UNEM)

For a more detailed description of plotting data in R, see the article on scatterplots.

Textxy

Within the calibrate package, the textxy() function can be used to label a plot’s data points. The textxy() function accepts the following arugments (“Label points in a plot,” n.d.).

Required

• x: the x values of the plot’s points
• y: the y values of the plot’s points
• labs: the labels to be associated with the plot’s points
• Optional

• cx: used to resize the label font
• dcol: used to set the label color; defaults to black
• m: sets the origin of the plot; defaults to (0,0)

Here, we will use textxy() to add labels for the enrollment at the University of New Mexico to each of our plot’s data points.

1. > #if necessary, install the calibrate package
2. > #install.packages(“calibrate”)
3. > #load the calibrate package
4. > library(calibrate)
5. > #use the textxy() function to add labels to the preexisting plot’s points
6. > #add labels for the total enrollment
7. > textxy(enrollmentData\$YEAR, enrollmentData\$UNEM, enrollmentData\$ROLL)

In this case, adding labels to our data points helps us to better assess the relationships in our dataset.

Complete Data Point Labeling Example

To see a complete example of how a plot’s data points can be labeled in R, please download the Data Point Labeling (.txt) file.

References

Label points in a plot. (n.d.). Retrieved September 19, 2010 from http://rss.acs.unt.edu/Rdoc/library/calibrate/html/textxy.html
Office of Institutional Research (1990). Enrollment Forecast [Data File]. Retrieved November 22, 2009 from http://lib.stat.cmu.edu/DASL/Datafiles/enrolldat.html

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