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Statistics LibreTexts

7.4: Plotting the Distribution of a Single Variable

  • Page ID
    8742
  • How do you choose which geometry to use? ggplot allows you to choose from a number of geometries. This choice will determine what sort of plot you create. We will use the built-in mpg dataset, which contains fuel efficiency data for a number of different cars.

    7.4.1 Histogram

    The histogram shows the ogerall distribution of the data. Here we use the nclass.FD function to compute the optimal bin size.

    ggplot(mpg, aes(hwy)) +
      geom_histogram(bins = nclass.FD(mpg$hwy)) +
      xlab('Highway mileage') 

    file27.png

    Instead of creating discrete bins, we can look at relative density continuously.

    7.4.2 Density plot

    ggplot(mpg, aes(hwy)) +
      geom_density() +
      xlab('Highway mileage') 

    file28.png

    A note on defaults: The default statistic (or “stat”) underlying geom_density is called “density” – not surprising. The default stat for geom_histogram is “count”. What do you think would happen if you overrode the default and set stat="count"?

    ggplot(mpg, aes(hwy)) +
      geom_density(stat = "count")

    What we discover is that the geometric difference between geom_histogram and geom_density can actually be generalized. geom_histogram is a shortcut for working with geom_bar, and geom_density is a shortcut for working with geom_line.

    7.4.3 Bar vs. line plots

    ggplot(mpg, aes(hwy)) +
      geom_bar(stat = "count")

    file29.png

    Note that the geometry tells ggplot what kind of plot to use, and the statistic (stat) tells it what kind of summary to present.

    ggplot(mpg, aes(hwy)) +
      geom_line(stat = "density")

    file30.png