We’ve gone over each of the basic units in the grammar of graphics: data, geometries, and aesthetics. Let’s extend this new knowledge to create a new type of plot: the bar chart. Bar charts are great for showing the distribution of categorical data. Typically, one of the axes on a bar chart will have numerical values and the other will have the names of the different categories you wish to understand.
Let’s build a bar chart by using some of the R built-in datasets. These are data frames that you can readily access in your code to explore and create visualizations. They are handy because these built-in datasets usually include nicely distributed categorical data.
The geom_bar() layer adds a bar chart to the canvas. Typically when creating a bar chart, you assign an aes() aesthetic mapping with a single categorical value on the x axes and the aes() function will compute the count for each category and display the count values on the y axis.
Since we’re extending the grammar of graphics, let’s also learn about how to save our visuals as local image files.
The following code maps the count of each category in the Language column in a dataset of 100 popular books to a bar length and then saves the visualization as a .png file named "bar-example.png":
Note: The ggsave() function allows you to save visualizations as a local file with the name of your choice. It’s a useful function when developing visualizations locally.
The code above outputs the following plot:
Instructions
The mpg dataset in R is a built-in dataset describing fuel economy data from 1999 and 2008 for 38 popular models of cars and is included with ggplot.
Inspect the built-in dataset mpg by printing its head(). Take special note of the class column which describes vehicle class for the cars with a total of 7 types (compact, SUV, minivan etc.)
Create a variable bar that is equal to a ggplot() object with the mpg built-in dataset associated as its data argument.
We want to understand the breakdown of the types of vehicles in the dataset, so provide the canvas, or the ggplot() object with an aesthetic mapping aes() that makes the x axis represent the categorical values of the class column in the dataframe. ggplot2 will count each unique value in the class column and automagically designate that value to the y axis.
Add a geom_bar() layer to bar. Be sure to type bar after you’ve declared the variable and added the layer so that the plot can render in your R notebook output.
Let’s add some color to the bar chart, by adding an aes() aesthetic mapping to the geom_bar() layer that fills the color of each bar based on the class value.
Our plot could use some context, let’s add a title and a sub-title so that users can understand more about what we are displaying with this bar chart and the mpg dataset.
Use the labs() function to assign a new title that describes this plot is illustrating the Types of Vehicles and a subtitle describing the data as From fuel economy data for popular car models (1999-2008)