Add Text Indicating the Sample Size to a ggplot2 Plot

For a strip plot or scatterplot produced using the package ggplot2 (e.g., with geom_point), for each value on the \(x\)-axis, add text indicating the number of \(y\)-values for that particular \(x\)-value.

stat_n_text(mapping = NULL, data = NULL, 
    geom = ifelse(text.box, "label", "text"), 
    position = "identity", na.rm = FALSE, show.legend = NA, 
    inherit.aes = TRUE, y.pos = NULL, y.expand.factor = 0.1, 
    text.box = FALSE, alpha = 1, angle = 0, color = "black", 
    family = "", fontface = "plain", hjust = 0.5, 
    label.padding = ggplot2::unit(0.25, "lines"), 
    label.r = ggplot2::unit(0.15, "lines"), label.size = 0.25, 
    lineheight = 1.2, size = 4, vjust = 0.5, ...)

Arguments

mapping, data, position, na.rm, show.legend, inherit.aes

See the help file for geom_text.

geom

Character string indicating which geom to use to display the text. Setting geom="text" will use geom_text to display the text, and setting geom="label" will use geom_label to display the text. The default value is geom="text" unless the user sets text.box=TRUE.

y.pos

Numeric scalar indicating the \(y\)-position of the text (i.e., the value of the argument y that will be used in the call to geom_text or geom_label). The default value is y.pos=NULL, in which case y.pos is set to the minimum value of all \(y\)-values minus a proportion of the range of all \(y\)-values, where the proportion is determined by the argument y.expand.factor (see below).

y.expand.factor

For the case when y.pos=NULL, a numeric scalar indicating the proportion by which the range of all \(y\)-values should be multiplied by before subtracting this value from the minimum value of all \(y\)-values in order to compute the value of the argument y.pos (see above). The default value is y.expand.factor=0.1.

text.box

Logical scalar indicating whether to surround the text with a text box (i.e., whether to use geom_label instead of geom_text). This argument can be overridden by simply specifying the argument geom.

alpha, angle, color, family, fontface, hjust, vjust, lineheight, size

See the help file for geom_text and the vignette Aesthetic specifications at https://cran.r-project.org/package=ggplot2/vignettes/ggplot2-specs.html.

label.padding, label.r, label.size

See the help file for geom_text.

...

Other arguments passed on to layer.

Details

See the help file for geom_text for details about how geom_text and geom_label work.

See the vignette Extending ggplot2 at https://cran.r-project.org/package=ggplot2/vignettes/extending-ggplot2.html for information on how to create a new stat.

References

Wickham, H. (2016). ggplot2: Elegant Graphics for Data Analysis (Use R!). Second Edition. Springer.

Author

Steven P. Millard (EnvStats@ProbStatInfo.com)

Note

The function stat_n_text is called by the function geom_stripchart.

See also

geom_stripchart, stat_mean_sd_text, stat_median_iqr_text, stat_test_text, geom_text, geom_label.

Examples

  # First, load and attach the ggplot2 package.
  #--------------------------------------------

  library(ggplot2)

  #====================

  # Example 1:

  # Using the built-in data frame mtcars, 
  # plot miles per gallon vs. number of cylinders
  # using different colors for each level of the number of cylinders.
  #------------------------------------------------------------------

  p <- ggplot(mtcars, aes(x = factor(cyl), y = mpg, color = factor(cyl))) + 
    theme(legend.position = "none")

  p + geom_point() + 
    labs(x = "Number of Cylinders", y = "Miles per Gallon")



  # Now add the sample size for each level of cylinder.
  #----------------------------------------------------

  dev.new()
  p + geom_point() + 
    stat_n_text() + 
    labs(x = "Number of Cylinders", y = "Miles per Gallon")
  
  #==========

  # Example 2:

  # Repeat Example 1, but:
  # 1) facet by transmission type, 
  # 2) make the size of the text smaller.
  #--------------------------------------

  dev.new()
  p + geom_point() + 
    stat_n_text(size = 3) + 
    facet_wrap(~ am, labeller = label_both) +  
    labs(x = "Number of Cylinders", y = "Miles per Gallon")
 
  #==========

  # Example 3:

  # Repeat Example 1, but specify the y-position for the text.
  #-----------------------------------------------------------

  dev.new()
  p + geom_point() + 
    stat_n_text(y.pos = 5) + 
    labs(x = "Number of Cylinders", y = "Miles per Gallon")
 
  #==========

  # Example 4:

  # Repeat Example 1, but show the sample size in a text box.
  #----------------------------------------------------------

  dev.new()
  p + geom_point() + 
    stat_n_text(text.box = TRUE) + 
    labs(x = "Number of Cylinders", y = "Miles per Gallon")
 
  #==========

  # Example 5:

  # Repeat Example 1, but use the color brown for the text.
  #--------------------------------------------------------
  
  dev.new()
  p + geom_point() + 
    stat_n_text(color = "brown") + 
    labs(x = "Number of Cylinders", y = "Miles per Gallon")

  #==========

  # Example 6:

  # Repeat Example 1, but:
  # 1) use the same colors for the text that are used for each group, 
  # 2) use the bold monospaced font.
  #------------------------------------------------------------------
  
  mat <- ggplot_build(p)$data[[1]]
  group <- mat[, "group"]
  colors <- mat[match(1:max(group), group), "colour"]

  dev.new()
  p + geom_point() + 
    stat_n_text(color = colors, size = 5, 
      family = "mono", fontface = "bold") + 
    labs(x = "Number of Cylinders", y = "Miles per Gallon")

  #==========

  # Clean up
  #---------

  graphics.off()
  rm(p, mat, group, colors)