Line Chart

Before you start: vectors and data frames; row order.
Setup: Use R’s built-in data or the vectors supplied on this page. Run a base plot before adding layers.
Your check: Sort a series meaningfully and retain original group identifiers. Practise in the unit summary.

What is a Line Chart?

A line chart connects observations in a meaningful order, often time. Within each series, sort the horizontal variable before drawing the connecting lines. The order should represent a sequence or progression in the data; do not connect unordered categories merely because there are many of them. Several series can share a plot when their lines are clearly identified.

Info
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.

For general titles, axes, colours and symbols, use the plot-parameter reference. The short table below is a local lookup for type=; you do not need to repeat the full plotting introduction.

type= can take the following values:

type	description
p	points
l	lines
o	overplotted lines and points
b	points joined by lines
c	no visible points (empty) joined by lines
s, S	line-like stair steps
h	histogram-like vertical lines
n	does not produce any points or lines

Example with pressure:

For these numeric vectors, plot(x, y) draws points by default. Set type = "l" to connect the observations with lines. The following calls compare the available types using the same ordered temperature series:

plot(x = pressure$temperature, y = pressure$pressure)                  # 0.
plot(x = pressure$temperature, y = pressure$pressure, type = "p")      # 1.
plot(x = pressure$temperature, y = pressure$pressure, type = "l")      # 2.
plot(x = pressure$temperature, y = pressure$pressure, type = "o")      # 3.
plot(x = pressure$temperature, y = pressure$pressure, type = "b")      # 4.
plot(x = pressure$temperature, y = pressure$pressure, type = "c")      # 5.
plot(x = pressure$temperature, y = pressure$pressure, type = "s")      # 6.
plot(x = pressure$temperature, y = pressure$pressure, type = "h")      # 7.
plot(x = pressure$temperature, y = pressure$pressure, type = "n")      # 8.

First row from left to right: 0. scatterplot without specification, 1. chart with points (equal to no specification), 2. line chart, 3. overplotted lines and point; second row from left to right: 4. points joined by lines, 5. no visible points (empty) joined by lines, 6. line-like stair steps: third row from left to right: 7. histogram-like vertical points, 8. no visible points or lines since it does not produce any.

To include multiple lines or to plot the points, first call plot() for the first line, then add additional lines and points with lines() and points() respectively:

# base graphic
plot(x = pressure$temperature, y = pressure$pressure,
  type = "l", col = "steelblue")

# add points
points(x = pressure$temperature, y = pressure$pressure, col = "steelblue")

# add second line in dark green
lines(x = pressure$temperature, y = pressure$pressure/2, col = "darkgreen")

# add points to second line
points(x = pressure$temperature, y = pressure$pressure/2, col = "darkgreen")

Line chart with multiple colored lines.

Example with Orange:

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. Tree is an ordered factor: its level order is 3, 1, 5, 2, 4, not numeric label order. Converting it directly with as.numeric() would return internal codes and relabel the trees. Keep the labels for selection and for the legend:

# Keep the original tree labels; use i only as a position
trees <- levels(Orange$Tree)
ntrees <- length(trees)

# 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 <- seq_len(ntrees)
plotchar <- 17 + seq_len(ntrees)

# add lines via for-loop
for (i in seq_along(trees))
{
  tree <- Orange[Orange$Tree == trees[i], ]
  tree <- tree[order(tree$age), ]
  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")


# add a legend
legend(xrange[1], yrange[2], legend=trees, cex=0.8, col=colors,
       pch=plotchar, lty=linetype, title="Tree")

The existing animation below illustrates the drawing steps using loop-position labels 1–5. Those positions correspond to original tree labels 3, 1, 5, 2 and 4, respectively. The corrected code above displays the original tree labels in its legend.

Line chart with multiple colored lines.