PCH in R Best Tips

25 Dec

PCH in R Best Tips

This article describes the different pch in R for modifying the point symbols of an R base plot. The option pch is used to specify point symbols in the functions plot() and lines().

In this tutorial, you’ll learn how to:

Display easily the list of pch in R. The R function ggpubr::show_point_shapes() can be used to show the 25 commonly used R pch values.
Change the R base plot pch symbols and appearance. Additionally, we provide R codes to modify the plot pch size and pch color, as well as, the legend pch. key arguments in plot() function:
- pch: numeric values (from 0 to 25) or character symbols (“+”, “.”, “;”, etc) specifying the point symbols (or shapes).
- cex: numeric values indicating the point size.
- col: color name for points.
Control pch types by groups.
Use special pch types, including pch 21 and pch 24. The interesting feature of these pch codes is that you can change their background fill color and, their border line type and color.

Contents:

Key R functions
List of pch symbols
Demo dataset
Change R base plot point shapes
Conclusion

Key R functions

plot(x, y, pch = 19, col = "black", cex = 1): Base R plot function to create a scatter plot.

List of pch symbols

The most commonly used pch values in R, include:

pch = 0, square
pch = 1, circle
pch = 2, triangle point up
pch = 3, plus
pch = 4, cross
pch = 5, diamond
pch = 6, triangle point down
pch = 7, square cross
pch = 8, star
pch = 9, diamond plus
pch = 10, circle plus
pch = 11, triangles up and down
pch = 12, square plus
pch = 13, circle cross
pch = 14, square and triangle down
pch = 15, filled square
pch = 16, filled circle
pch = 17, filled triangle point-up
pch = 18, filled diamond
pch = 19, solid circle
pch = 20, bullet (smaller circle)
pch = 21, filled circle blue
pch = 22, filled square blue
pch = 23, filled diamond blue
pch = 24, filled triangle point-up blue
pch = 25, filled triangle point down blue

The function below illustrates the different pch values. First install the ggpubr package (install.packages("ggpubr")), and then type this:

ggpubr::show_point_shapes()

Note that,

points can be omitted from the plot using pch = NA.
Other different characters symbols can be used to specify the pch argument, including “+”, “*“,”-“,”.“,”#, “%”, “o”.
pch options from pch 21 to 25 are open symbols that can be filled by a color.

Demo dataset

We’ll use the R built-in datasets: iris.

head(iris, 3)

##   Sepal.Length Sepal.Width Petal.Length Petal.Width Species
## 1          5.1         3.5          1.4         0.2  setosa
## 2          4.9         3.0          1.4         0.2  setosa
## 3          4.7         3.2          1.3         0.2  setosa

Change R base plot point shapes

The default R plot pch symbol is 1, which is an empty circle. You can change this to pch = 19 (solid circle) or to pch = 21 (filled circle).

For example:

# Default plot pch = 1 (empty circle)
plot(x = iris$Sepal.Length, y = iris$Sepal.Width, frame = FALSE,
     xlab = "Sepal Length", ylab = "Sepal Width")

# Change plot symbol to pch = 19 (solid circle)
plot(x = iris$Sepal.Length, y = iris$Sepal.Width, frame = FALSE,
     xlab = "Sepal Length", ylab = "Sepal Width",
     pch = 19)

To change the color and the size of points, use the following arguments:

col: color (hexadecimal color code or color name). For example, col = "blue" or col = "#4F6228".
cex: the size of point symbols. Numeric values.

For the filled pch symbols (21 to 25), you can use additional plot options to modify the points background color (bg) and the border line width (lwd):

bg: the background (or fill) color for the open plot symbols (21 to 25).
lwd: Numeric values indicating the line width of the plotting symbols border.

# Change color
plot(x = iris$Sepal.Length, y = iris$Sepal.Width, frame = FALSE,
     xlab = "Sepal Length", ylab = "Sepal Width",
     pch = 19, col =  "#0073C2FF")

# Use pch = 21
# Change border line width (lwd), and background color (bg)
plot(x = iris$Sepal.Length, y = iris$Sepal.Width, frame = FALSE,
     xlab = "Sepal Length", ylab = "Sepal Width",
     pch = 21, bg = "lightgray", col = "black", 
     lwd = 0.9, cex = 1.5)

Note that point color and pch can be also a vector. For example, you might want to change point colors and shapes by groups:

# Define color for each of the 3 iris species
colors <- c("#00AFBB", "#E7B800", "#FC4E07")
colors <- colors[as.numeric(iris$Species)]

# Define shapes
shapes = c(16, 17, 18) 
shapes <- shapes[as.numeric(iris$Species)]

# Plot
plot(x = iris$Sepal.Length, y = iris$Sepal.Width, frame = FALSE,
     xlab = "Sepal Length", ylab = "Sepal Width",
     col = colors, pch = shapes)
legend("topright", legend = levels(iris$Species),
      col =  c("#00AFBB", "#E7B800", "#FC4E07"),
      pch = c(16, 17, 18) )

Conclusion

This article describes the different point shapes (or pch symbols) available in R.

Display the different point symbols in R:

ggpubr::show_point_shapes()

Change point symbols in R base plots. Use pch, cex and col to change, respectively, the symbols, the size and the color of points in R base plots:

plot(x = iris$Sepal.Length, y = iris$Sepal.Width, frame = FALSE,
     xlab = "Sepal Length", ylab = "Sepal Width",
     pch = 19, cex = 1, col = "#00AFBB")