About plotting in R
Devices
R handles plots via so-called Graphic Devices. These devices are outlets that we can use to do different things with the plotting instructions: for instance, we can either display them on the computer screen, or we can write them directly into files.
We will look into devices a bit later - suffice it to say that if you do not specify what device you want to use, a default device will be opened for you, which will render your plot on the computer screen (either separate window or in a panel in RStudio (RStudioGD)).
Function level in plotting
In traditional (S-style) R we usually differentiate between high-level and low-level plotting functions.
High-level plotting functions
High-level plotting functions create entire plots based on the given arguments. These functions
- finalize (or erase) the existing plot,
- create a new plot for us to work with,
- and Draw someting on the plot for us, by calling to some low-level plotting functions themselves.
The most frequently used high-level plotting function in R is plot(). This function can be used to visualize most objects in R, and in multiple ways.
This call will make a very simple - but complete plot:
plot(x=0, y=0)
High-level plotting functions are designed to work with structured data, usually a large set of values. Such functions include histograms (hist()) and boxplots (boxplot()).
Low-level plotting functions
Low-level plotting functions add elements to existing plots. These can include simple geometries, text, curves, polygons - even existing images.
Most high-level plotting functions have some low-level plotting functionality in them. For instance, the call above to plot() does multiple things, that you can also invoke separately:
- It initializes the plot (
plot.new()) and sets the parameters (par()) - It draws the x and y axes (
axis()). - It draws the box around the plot (
box()) - It draws x and y axis labels (
mtext()) - It adds a single point at (0,0) (
points())
To illustrate the concept and how to control these aspects separately, you can recreate the plot above like this. Execute these lines one-by-one to see how this plot can be built up from parts:
plot(0,0, axes=FALSE, ylab="", xlab="", type="n") # start the plot
box() # draw a box around plotting area
axis(1) # draw horizontal axis, side 1: bottom
axis(2) # draw vertical axis, side 2: left
mtext(text="0",side=1, line=3) # draw text beyond the plotting margin
mtext(text="0", side=2, line=3) # draw text beyond the plotting margin
points(0,0) # add single point
Note that we are intentionally suppressing the default plotting behaviour!