An effective and accurate data visualization is an important part of a statistical analysis. It can make your data come to life and convey your message in a powerful way.
R has very strong graphics capabilities that can help you visualize your data.
The plot() function
In R, the base graphics function to create a plot is the
plot() function. It has many options and arguments to control many things, such as the plot type, labels, titles and colors.
The syntax for the
plot() function is:
|x||The coordinates of points in the plot|
|y||The y coordinates of points in the plot|
|type||The type of plot to be drawn|
|main||An overall title for the plot|
|xlab||The label for the x axis|
|ylab||The label for the y axis|
|pch||The shape of points|
|col||The foreground color of symbols as well as lines|
|las||The axes label style|
|bty||The type of box round the plot area|
|bg||The background color of symbols (only 21 through 25)|
|cex||The amount of scaling plotting text and symbols|
|…||Other graphical parameters|
Create a Simple Plot
To get started with plot, you need a set of data to work with.
Let’s consider the built-in pressure dataset as an example dataset. It contains observations of the vapor pressure of mercury over a range of temperatures.
# First six observations of the 'Pressure' dataset head(pressure) temperature pressure 1 0 0.0002 2 20 0.0012 3 40 0.0060 4 60 0.0300 5 80 0.0900 6 100 0.2700
To create a plot just specify the dataset in
# Plot the 'pressure' dataset plot(pressure)
Change the Shape and Size of the Points
You can use the pch (plotting character) argument to specify symbols to use when plotting points.
Here’s a list of symbols you can use.
# Change the shape of the points plot(pressure, pch=17)
For symbols 21 through 25, you can specify border color using col argument and fill color using bg argument.
# Change the border color to blue and background color to lightblue plot(pressure, pch=21, col="blue", bg="lightblue")
To alter the size of the plotted characters, use cex (character expansion) argument.
# Scale the data points by 1.2 plot(pressure, cex=1.2)
Changing the Color
You can change the foreground color of symbols using the argument col.
# Change the color of symbols to red plot(pressure, col="red")
R has a number of predefined colors that you can use in graphics. Use the
colors() function to get a complete list of available names for colors.
# List of predefined colors in R colors()  "white" "aliceblue" "antiquewhite"  "antiquewhite1" "antiquewhite2" "antiquewhite3" ...
Or you can refer the following color chart.
You can specify colors by index, name, hexadecimal, or RGB value. For example
col="#FFFFFF" are equivalent.
Different Plot Types
You can change the type of plot that gets drawn by using the type argument.
Here’s a list of all the different types that you can use.
|“b”||Both points and lines|
|“c”||The lines part alone of “b”|
|“o”||Both points and lines “overplotted”|
|“h”||Histogram like (or high‐density) vertical lines|
|“s”||Step plot (horizontal first)|
|“S”||Step plot (vertical first)|
For example, to create a plot with lines between data points, use
type="l"; to draw both lines and points, use
A series of graphics showing different types is shown below.
Adding Titles and Axis Labels
You can add your own title and axis labels easily by specifying following arguments.
|main||Main plot title|
plot(pressure, main = "Vapor Pressure of Mercury", xlab = "Temperature (deg C)", ylab = "Pressure (mm of Hg)")
The Axes Label Style
By specifying the las (label style) argument, you can change the axes label style. This changes the orientation angle of the labels.
|0||The default, parallel to the axis|
|2||Perpendicular to the axis|
For example, to change the axis style to have all the axes text horizontal, use
plot(pressure, las = 1)
The Box Type
Specify the bty (box type) argument to change the type of box round the plot area.
|“o”||(default) Draws a complete rectangle around the plot.|
|“n”||Draws nothing around the plot.|
|“l”, “7”, “c”, “u”, or “]”||Draws a shape around the plot area.|
# Remove the box round the plot plot(pressure, bty="n")
Add a Grid
plot() function does not automatically draw a grid. However, it is helpful to the viewer for some plots. Call the
grid() function to draw the grid once you call the
Add a Legend
You can include a legend to your plot – a little box that decodes the graphic for the viewer. Call the
legend() function, once you call the
# Add a legend to the top left corner plot(pressure, col="red", pch=19) points(pressure$temperature/2, pressure$pressure,col="blue", pch=17) legend("topleft", c("line 1","line 2"), pch=c(19,17), col=c("red","blue"))
The position of the legend can be specified using the following keywords : “bottomright”, “bottom”, “bottomleft”, “left”, “topleft”, “top”, “topright”, “right” and “center”.
The effect of using each of these keywords is shown below.
Add Points to a Plot
You can add points to a plot with the
For example, let’s create a subset of pressure containing temperatures greater than 200 °C and add these points to the plot.
plot(pressure, col = "red") points(pressure[pressure$temperature > 200, ], col = "red", pch = 19)
Add Lines to a Plot
You can add lines to a plot in a very similar way to adding points, except that you use the
lines() function to achieve this.
plot(pressure) lines(pressure$temperature/2, pressure$pressure)
You can change the line type using lty argument; and the line width using lwd argument.
# Change the line type and line width plot(pressure) lines(pressure$temperature/2, pressure$pressure, lwd=2, lty=3)
Here’s a list of line types you can use.
There’s another function called
abline() which allows you to draw horizontal, vertical, or sloped lines.
# Draw a dotted horizontal line at 247 and vertical line at 300 plot(pressure) abline(h= 247, v=300, col="red", lty=2)
Label Data Points
text() function to add text labels at any position on the plot.
The position of the text is specified by the pos argument. Values of 1, 2, 3 and 4, respectively places the text below, to the left of, above and to the right of the specified coordinates.
# Add text labels above the coordinates plot(pressure, pch=19, col="red") text(pressure, labels=pressure$pressure, cex=0.7, pos=3, col="blue")
Set Axis Limits
By default, the
plot() function works out the best size and scale of each axis to fit the plotting area. However, you can set the limits of each axis quite easily using xlim and ylim arguments.
# Change the axis limits so that the x-axis and y-axis ranges from 0 to 500 plot(pressure, ylim=c(0,500), xlim=c(0,500))
Display Multiple Plots on a Single Page
By using the
mfrow graphics parameter, you can display multiple plots on the same graphics page.
To use this parameter, you need to pass a two-element vector, specifying the number of rows and columns. Then fill each cell in the matrix by repeatedly calling plot.
mfrow=c(1, 2) creates two side by side plots.
par(mfrow = c(1, 2)) plot(cars, main="Speed vs Distance", col="red") plot(mtcars$mpg, mtcars$hp, main="HP vs MPG", col="blue")
Once your plot is complete, you need to reset your
par() options. Otherwise, all your subsequent plots will appear side by side.
# Reset the mfrow parameter par(mfrow = c(1,1))
Save a Plot to an Image File
To save a plot to an image file, you have to do three things in sequence:
- Call a function to open a new graphics file, such as
plot()to generate the graphics image.
dev.off()to close the graphics file.
# Save a plot as a png file png(filename="myPlot.png", width=648, height=432) plot(pressure, col="slateblue1", pch=19, type="b", main = "Vapor Pressure of Mercury", xlab = "Temperature (deg C)", ylab = "Pressure (mm of Hg)") dev.off()
Remember that the file will be saved to your current working directory, unless you specify an absolute file path.