Line Types in R: The Ultimate Guide for R Base Plot and GGPLOT

Key R functions

• plot(x, y, type = "b", lty = 1, lwd = 1, col = "blue") and lines(x, y, lty = 2, lwd = 1): Base R plot functions to create line plots.
• geom_line(aes(x, y), data = NULL, linetype = "dashed", color = "black", size = 1): ggplot2 function to create line plots.
• scale_linetype_manual(), scale_color_manual() and scale_size_manual(): ggplot2 functions to set manually lines type, color and size.

Show the different line types in R

The list of line types available in R, includes:

0. “blank”, 1. “solid”, 2. “dashed”, 3. “dotted”, 4. “dotdash”, 5. “longdash” and 6. “twodash”.

The graph below illustrates the list of line types available in R:

library(ggpubr)
show_line_types()

Change R base plot line types

Simple format of R lines functions:

• plot(x, y, type = "l", lty = 1). Create the main R base plot frame.
• lines(x, y, type = "l", lty = 1). Add lines onto the plot.

Key options:

• x, y: variables to be used for the x and y axes, respectively.
• type: display the data as line and/or point. Lowed values: l (display line only), p (show point only) and b (show both).
• pch and cex: set points shape and size, respectively.
• lty, lwd: set line types and thickness.
• col: change the color of point and line.
• xlab and ylab: for x and y axis labels, respectively.

Create a plot with multiple lines and set the legend lty. We start by plotting a first single line with a solid line type (lty = 1). Next, we add a second line with a dashed line style (lty = 2). Finally, we add a legend on the plot using the R base function legend(), which take the same col and lty arguments as the lines function. The option cex is used to set the legend text size.

# 1. Create some variables
x <- 1:10
y1 <- x*x
y2  <- 2*y1

# 2. Plot a first line
plot(x, y1, type = "b", frame = FALSE, pch = 19,
     col = "red", xlab = "x", ylab = "y", 
     lty = 1, lwd = 1)

# 3. Add a second line
lines(x, y2, pch = 18, col = "blue", type = "b", 
      lty = 2, lwd = 1)

# 4. Add a legend to the plot and set legend lty
legend("topleft", legend = c("Line 1", "Line 2"),
       col = c("red", "blue"), lty = 1:2, cex = 0.8)

Change ggplot line types

Data set: ToothGrowth

ToothGrowth$dose <- as.factor(ToothGrowth$dose)
head(ToothGrowth, 3)

##    len supp dose
## 1  4.2   VC  0.5
## 2 11.5   VC  0.5
## 3  7.3   VC  0.5

1. Basic line plot showing the mean value of tooth length (len) by the dose of vitamin C (dose):

• Compute the mean of tooth length grouped by dose:

library(dplyr)
df <- ToothGrowth %>%
  group_by(dose) %>%
  summarise(len.mean = mean(len))
df

## # A tibble: 3 x 2
##   dose  len.mean
##   <fct>    <dbl>
## 1 0.5       10.6
## 2 1         19.7
## 3 2         26.1

• Create a single line plot. Change the linetype option to “dashed”.

library(ggplot2)
ggplot(data = df, aes(x = dose, y = len.mean, group = 1)) +
  geom_line(linetype = "dashed")+
  geom_point()

2. Create a line plot for multiple groups. Change line types by groups.

• Data:

# Compute the mean of `len` grouped by dose and supp
library(dplyr)
df2 <- ToothGrowth %>%
  group_by(dose, supp) %>%
  summarise(len.mean = mean(len))
df2

## # A tibble: 6 x 3
## # Groups:   dose [?]
##   dose  supp  len.mean
##   <fct> <fct>    <dbl>
## 1 0.5   OJ       13.2 
## 2 0.5   VC        7.98
## 3 1     OJ       22.7 
## 4 1     VC       16.8 
## 5 2     OJ       26.1 
## 6 2     VC       26.1

• Line plot:

# Change line types by groups
ggplot(df2, aes(x = dose, y = len.mean, group = supp)) +
  geom_line(aes(linetype = supp))+
  geom_point()+
  theme(legend.position = "top")

# Change line types + colors by groups 
ggplot(df2, aes(x = dose, y = len.mean, group = supp)) +
  geom_line(aes(linetype = supp, color = supp))+
  geom_point(aes(color = supp))+
  theme(legend.position = "top")

3. Change the appearance of line types manually:

• scale_linetype_manual(): change line types
• scale_color_manual(): change line colors
• scale_size_manual(): change the size of lines

To be able to apply these functions, you should create a geom_line, which line types, color and size should be controlled by groups.

# Change manually line type and color manually
ggplot(df2, aes(x = dose, y = len.mean, group = supp)) +
  geom_line(aes(linetype = supp, color = supp))+
  geom_point(aes(color = supp))+
  scale_linetype_manual(values=c("solid", "dashed"))+
  scale_color_manual(values=c("#00AFBB","#FC4E07"))

Conclusion

• Use lty and lwd options, for changing lines type and thickness in R base graphics:

x <- 1:10; y1 <- x*x; y2  <- 2*y1
# Draw two lines
plot(x, y1, type = "b", pch = 19, col = "red", lty = 1, lwd = 1)
lines(x, y2, pch = 18, col = "blue", type = "b", lty = 2, lwd = 1)
# Add legend
legend("topleft", legend = c("Line 1", "Line 2"),
       col = c("red", "blue"), lty = 1:2)

• Use linetype and size arguments in ggplot2 :

# Create some data
x <- 1:10; y1 <- x*x; y2  <- 2*y1
df <- data.frame(
  x = c(x, x), y = c(y1, y2),
  grp = as.factor(rep(c("A", "B"), each = 10))
  )
# Plot
library(ggplot2)
ggplot(data = df, aes(x, y, group = grp)) +
  geom_line(aes(linetype = grp))+
  geom_point()