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.

Syntax

The syntax for the plot() function is:

plot(x,y,type,main,xlab,ylab,pch,col,las,bty,bg,cex,…)

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() function.

# 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()
[1] "white"         "aliceblue"     "antiquewhite" 
[4] "antiquewhite1" "antiquewhite2" "antiquewhite3"
...

Or you can refer the following color chart.

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.

For example, to create a plot with lines between data points, use type="l"; to draw both lines and points, use type="b".

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.

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.

For example, to change the axis style to have all the axes text horizontal, use las=1

plot(pressure, las = 1)

The Box Type

Specify the bty (box type) argument to change the type of box round the plot area.

# Remove the box round the plot
plot(pressure, bty="n")

Add a Grid

The 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 plot().

plot(pressure)
grid()

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 plot().

# 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 points() function.

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

Use the 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.

For example, 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 png(), jpg() or pdf().
• Call plot() to generate the graphics image.
• Call 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()