Introduction
In this tutorial you won’t make any plots, but you will learn the logic that builds up all plots in R. So please use this explanation together with another data visualization tutorial that shows the types of plots most relevant to your data.
The logic of ggplot
ggplot2 is a powerful R package for creating figures. It is a part of
the tidyverse collection, so to use ggplot, you need to load
tidyverse. Figures made with ggplot are built from several layers.
You always use the same basic code structure to create a wide range of
figures:
- The
ggplot()function creates a blank canvas for you to work on. - Geoms add the visual elements, such as points, lines, bars, or other shapes.
- Other specifications can include changing axis settings, setting the theme, adding labels, etc.
- You connect all these different specifications to each other using
+signs.
The variables that you want to display on the graph must always be
wrapped in an aes() function, which stands for aesthetics. This
specification tells R to determine the value of the aesthetic (x and y
axes, colors, groups, line types, etc.) based on the value of the
variable. aes() can be specified both in the main ggplot() function
(in which case it will apply to all geoms) or within a geom_...()
function (then it only applies to that geom).
For line charts you use geom_line(), and for scatterplots
geom_point(). You can fit a line on your scatterplot with
geom_smooth(). If you want to fit a straight line, add method = "lm"
as an argument inside geom_smooth().
If you want to change the color of a point/line based on the value of a
variable, specify color = variable inside the aes() function. If
you would like all points/lines to be a particular color, specify
color = "blue" outside the aes() function but still inside the
geom you’d like to modify. Each geom’s help file lists all
characteristics that you can modify.
Saving plots
Once you make a plot, you can save ggplot objects to use outside of
the R environment with the ggsave function. You can specify an
existing ggplot object as the first argument, but by default
ggsave() saves the last plot displayed in your Plots panel. You always
need to specify the file path of the saved plot, including the preferred
file format (e.g. .png, .jpg, .pdf). You can adjust the plot size with
the scale argument or by specifying the height and width in your
preferred units (the default units are inches).
# Save last plot
ggsave("figures/plot1.png", scale = 1.5)