How to make QQ plot in R

Dear friends,

For those of you that are still having some problems making chart in R, here’s an example for QQ plot. I made it steps.

1. Read data

Use your own data.

QQTest <- read.table(file="QQTest.dat",sep=",",head=TRUE)

2. Make a QQplot

qqnorm(QQTest$vals)

3. Annotate the plot

qqnorm(QQTest$vals,
       main="Normal Q-Q Plot of ...",
       xlab="Observed",
       ylab="Modeled")

4. Make the same x and y limit

qqnorm(QQTest$vals,
       main = "Normal Q-Q Plot of ...",
       xlab = "Observed",
       ylab = "Modeled",
       xlim = c(-3, 3),
       ylim = c(-3, 3)) 

5. Change to + symbol

qqnorm(QQTest$vals,
       main = "Normal Q-Q Plot of ...",
       xlab = "Observed",
       ylab = "Modeled",
       xlim = c(-3, 3),
       ylim = c(-3, 3),
       pch = "+") 

Add the QQ line

qqline(QQTest$vals, col = "blue", lwd = 2, lty = 2)

Or make it red

qqline(QQTest$vals, col = "red", lwd = 2, lty = 2)

Or make it not dotted

qqline(QQTest$vals, col = "blue", lwd = 2, lty = 1)

Or make it thinner

qqline(QQTest$vals, col = "blue", lwd = 1, lty = 1)

6. Add the grid

qqnorm(QQTest$vals,
       main = "Normal Q-Q Plot of ...",
       xlab = "Observed",
       ylab = "Modeled",
       xlim = c(-3, 3),
       ylim = c(-3, 3),
       pch = "+",
       panel.first = grid(7, lty = 1, lwd = 2)) 

Not good right?

Try this one

qqnorm(QQTest$vals,
       main = "Normal Q-Q Plot of ...",
       xlab = "Observed",
       ylab = "Modeled",
       xlim = c(-3, 3),
       ylim = c(-3, 3),
       pch = "+") 
abline(v=(seq(-3, 3, by=1)), col="lightgray", lty="dotted")
abline(h=(seq(-3, 3, by=1)), col="lightgray", lty="dotted")

7. Make two columns plot: side by side

First delete your old plot using plot.new() then change the layout

plot.new()
par(mfrow=c(2, 1))

Make the left plot

qqnorm(QQTest$vals,
       main = "Upper Plot -> Normal Q-Q Plot of ...",
       xlab = "Observed",
       ylab = "Modeled",
       xlim = c(-3, 3),
       ylim = c(-3, 3),
       pch = "+") 
abline(v=(seq(-3, 3, by=1)), col="lightgray", lty="dotted")
abline(h=(seq(-3, 3, by=1)), col="lightgray", lty="dotted")

Then make the right plot

qqnorm(QQTest$vals,
       main = "Right Plot -> Normal Q-Q Plot of ...",
       xlab = "Observed",
       ylab = "Modeled",
       xlim = c(-3, 3),
       ylim = c(-3, 3),
       pch = "+") 
abline(v=(seq(-3, 3, by=1)), col="lightgray", lty="dotted")
abline(h=(seq(-3, 3, by=1)), col="lightgray", lty="dotted")

Or make it two rows: upper and lower

plot.new()
par(mfrow=c(1, 2))

Make the upper plot

qqnorm(QQTest$vals,
       main = "Upper Plot -> Normal Q-Q Plot of ...",
       xlab = "Observed",
       ylab = "Modeled",
       xlim = c(-3, 3),
       ylim = c(-3, 3),
       pch = "+") 
abline(v=(seq(-3, 3, by=1)), col="lightgray", lty="dotted")
abline(h=(seq(-3, 3, by=1)), col="lightgray", lty="dotted")

Then make the lower plot

qqnorm(QQTest$vals,
       main = "Lower Plot -> Normal Q-Q Plot of ...",
       xlab = "Observed",
       ylab = "Modeled",
       xlim = c(-3, 3),
       ylim = c(-3, 3),
       pch = "+") 
abline(v=(seq(-3, 3, by=1)), col="lightgray", lty="dotted")
abline(h=(seq(-3, 3, by=1)), col="lightgray", lty="dotted")

There you go. You make more tweaks it based on your needs. You can also use a specific package with specific style like: lattice or ggplot2. Make sure you read the documentation first, as each package may have different commands, but the basic principle is the same.

Good luck.

@dasaptaerwin