Quantile-Quantile Plots

This is a re-write of the QQ-plotting functions provided by stats, using the ggplot2 library. qqnorm is a generic function the default method of which produces a normal QQ plot of the values in y. qqline adds a line to a “theoretical”, by default normal, quantile-quantile plot which passes through the probs quantiles, by default the first and third quartiles. qqplot produces a QQ plot of two datasets.

qqnorm(
  y,
  ylim,
  main = "Normal Q-Q Plot",
  xlab = "Theoretical Quantiles",
  ylab = "Sample Quantiles",
  datax = FALSE
)

qqplot(
  x,
  y,
  xlab = deparse(substitute(x)),
  ylab = deparse(substitute(y)),
  main = "Q-Q Plot"
)

qqline(
  y,
  datax = FALSE,
  distribution = qnorm,
  probs = c(0.25, 0.75),
  qtype = 7,
  ...
)

Arguments

y,	The second or only data sample.
ylim, ...,	Graphical parameters.
main, xlab, ylab,	Plot labels. The xlab and ylab refer to the y and x axes respectively if datax = TRUE.
datax,	Logical. Should data values be on the x-axis ?
x,	The first sample for qqplot.
distribution	quantile function for reference theoretical distribution.
probs	numeric vector of length two, representing probabilities. Corresponding quantile pairs define the line drawn.
qtype	the type of quantile computation used in quantile.

Note

Help file is mainly from stats::qqnorm since this is a rewrite of stats::qqplot, stats::qqline and stats::qqnorm using the ggplot2 library.

Examples

library(mgcViz)
y <- rt(500, df = 5)

# Compare new and old version of qqnorm
stats::qqnorm(y)
qqnorm(y)

# Compare new and old version of qqplot
x <- rt(200, df = 5)
y <- rt(300, df = 5)
stats::qqplot(x, y)
qqplot(x, y)
# add a qqline()
ggplot2::last_plot() + qqline(y = rt(500, df = 4.8), col = "green")

## "QQ-Chisquare" : --------------------------
y <- rchisq(500, df = 3)
## Q-Q plot for Chi^2 data against true theoretical distribution:
x <- qchisq(ppoints(500), df = 3)
stats::qqplot(qchisq(ppoints(500), df = 3), rchisq(500, df = 3),
      main = expression("Q-Q plot for" ~~ {chi^2}[nu == 3]))
qqplot(qchisq(ppoints(500), df = 3), rchisq(500, df = 3),
      main = expression("Q-Q plot for" ~~ {chi^2}[nu == 3])) + theme_bw()