ggplot

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ggplot

  • Very popular plotting package
  • Good plots quickly
  • Declarative - describe what you want not how to build it
  • Contrasts w/Imperative - how to build it step by step

Basics

  • Load the package and some data

    

  

  

  





    

  • To build a plot using ggplot we start with the ggplot() function
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  • ggplot() creates a base ggplot object that we can then add things to - like a blank canvas
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  • We can also add optional arguments for information to be shared across different components of the plot
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  • The two main arguments we typically use here are
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  • data - which is the name of the data frame we are working with, so acacia
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  • mapping - which describes which columns of the data are used for different aspects of the plot
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  • We create a mapping by using the aes function, which stands for “aesthetic”, and then linking columns to pieces of the plot
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  • We’ll start with telling ggplot what value should be on the x and y axes
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  • Let’s plot the relationship betwen the circumference of an acacia and its height
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  • This still doesn’t create a figure, it’s just a blank canvas and some information on default values for data and mapping columns to pieces of the plot
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  • We can add data to the plot using layers
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  • We do this by adding a + after the the ggplot function and then adding something called a geom, which stands for geometry
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  • To make a scatter plot we use geom_point
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  • To change things about the layer we can pass additional arguments to the geom
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  • We can do things like change

      

    • the size of the points, we’ll set it to 3
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    • the color of the points, we’ll set it to "blue"
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    • the transparency of the points, which is called alpha, we’ll set it to 0.5
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  • Try changing these values to make the graph look like you want it to
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  • To add labels (like documentation for your graphs!) we use the labs function
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Grouping


    

  • Group on a single graph
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  • Look at influence of experimental treatment
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  • Try changing the above code to color based on the gear
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  • We can also split each group into different subplots (known as facets)
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  • Try changing this code to create a subplot for each value in vs with points of size 4
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Exercise







Make a scatter plot with hp on the x axis and wt on the y axis. Label the x axis “Horse Power” and the y axis “Weight”. Make one subplot for each value in gear.



  
  

    

    

      

    

  

  

  





Your result should look like the plot below






































library(ggplot2)
ggplot(mtcars, aes(x = hp, y = wt)) +
  geom_point() +
  labs(x = "Horse Power", y = "Weight") +
  facet_wrap(~gear)