How To Create A Line Graph In R

Line Charts

Overview

Line charts are created with the function lines( x , y , type=) where x and y are numeric vectors of (x,y) points to connect. type= can take the following values:

type	description
p	points
l	lines
o	overplotted points and lines
b, c	points (empty if "c") joined by lines
s, S	stair steps
h	histogram-like vertical lines
n	does not produce any points or lines

The lines( ) function adds information to a graph. It can not produce a graph on its own. Usually it follows a plot( x , y ) command that produces a graph.

By default, plot( ) plots the (x,y) points. Use the type="n" option in the plot( ) command, to create the graph with axes, titles, etc., but without plotting the points.

(To practice creating line charts with this lines( ) function, try this exercise.)

Example

In the following code each of the type= options is applied to the same dataset. The plot( ) command sets up the graph, but does not plot the points.

x <- c(1:5); y <- x # create some data par(pch=22, col="red") # plotting symbol and color par(mfrow=c(2,4)) # all plots on one page opts = c("p","l","o","b","c","s","S","h") for(i in 1:length(opts)){ heading = paste("type=",opts[i]) plot(x, y, type="n", main=heading) lines(x, y, type=opts[i]) }

lines options without points click to view

Next, we demonstrate each of the type= options when plot( ) sets up the graph and does plot the points.

x <- c(1:5); y <- x # create some data par(pch=22, col="blue") # plotting symbol and color par(mfrow=c(2,4)) # all plots on one page opts = c("p","l","o","b","c","s","S","h") for(i in 1:length(opts){ heading = paste("type=",opts[i]) plot(x, y, main=heading) lines(x, y, type=opts[i]) }

lines options with points click to view

As you can see, the type="c" option only looks different from the type="b" option if the plotting of points is suppressed in the plot( ) command.

To demonstrate the creation of a more complex line chart, let's plot the growth of 5 orange trees over time. Each tree will have its own distinctive line. The data come from the dataset Orange.

# Create Line Chart

# convert factor to numeric for convenience Orange$Tree <- as.numeric(Orange$Tree) ntrees <- max(Orange$Tree)

                                       # get the range for the x and y axis              
              xrange <- range(Orange$age)              
              yrange <- range(Orange$circumference)            
                                       # set up the plot              
              plot(xrange, yrange, type="n", xlab="Age (days)",
              ylab="Circumference (mm)" )              
              colors <- rainbow(ntrees)              
              linetype <- c(1:ntrees)              
              plotchar <- seq(18,18+ntrees,1)
                                       # add lines              
              for (i in 1:ntrees) {              
              tree <- subset(Orange, Tree==i)              
              lines(tree$age, tree$circumference, type="b", lwd=1.5,
              lty=linetype[i], col=colors[i], pch=plotchar[i])              
              }            
                                       # add a title and subtitle              
              title("Tree Growth", "example of line plot")
                                       # add a legend              
              legend(xrange[1], yrange[2], 1:ntrees, cex=0.8, col=colors,
              pch=plotchar, lty=linetype, title="Tree")

line chart click to view

Going Further

Try the exercises in this course on plotting and data visualization in R.