qqnorm
Normal Quantile-Quantile Plots

### Description

Produces data for a Normal Quantile-Quantile plot, which is plot of the order data values versus quantiles from a Normal distribution. The displayed points should follow a linear shape if the data values are from Normal distribution.
The qqnorm function is generic (see Methods); method functions can be written to handle specific classes of data.

### Usage

    qqnorm(x, ...)
    ## Default S3 method:
    qqnorm(y, ylim, main="Normal Q-Q Plot",
    	     xlab="Theoretical Quantiles", ylab="Sample Quantiles",
    	     plot.it=TRUE, datax = FALSE, ...)

### Arguments

y | a numeric vector for qqnorm.default, an object for qqnorm. Missing values, (NA), are allowed. There should be at least one value that is not (NA).
ylim | numeric of length 2 giving the y limits for the plot.
main | overall title for the plot.
xlab, ylab | character strings for axis labels. The default, if datax is TRUE, is the argument passed as x for the y-axis, and the argument passed as y for the x-axis. These are switched if datax is FALSE.
plot.it | if FALSE, qqplot.default and qqnorm return a non-visible list with components x and y. If TRUE, they return the coordinates of the points that would have been plotted.
datax | if TRUE, data y goes on the x-axis; if FALSE data y goes on the y-axis. And the axis labels will change at the same time.
... | Graphical parameters may also be supplied as arguments to this function (see par ). In particular these functions can take arguments type and log to control plot type and logarithmic axes (see plot.default ). Use the arguments xlim and ylim to control the limits of the plot region. In addition, the high-level graphics arguments described under plot.default and the arguments to title may be supplied to this function.

### Details

The function qqnorm takes a single vector of data and an optional vector of weights for a normal (Gaussian) probability plot. When plot.it is FALSE, it returns a non-visible list with 2 components. The component y comes from the argument Y with value NA is striped. The component x is the result of qnorm(ppoints(length(y)))[order(order(y))] where NA value in argument y has been striped, but the NA value in argument y is placed in the same position to the result components. And, if datax is TRUE, the components x and y should exchange its value.
Value
if plot.it is FALSE, qqnorm returns a non-visible list with components x and y. If plot.it is TRUE, returns the coordinates of the points that would have been plotted.
Side Effects
if plot.it=TRUE, a Normal quantile-quantile plot is produced on the current graphics device.
Background
QQplots are used to assess whether data have a particular distribution, or whether two datasets have the same distribution. If the distributions are the same, then the plot will be approximately a straight line. The extreme points have more variability than points toward the center. A plot with a "U" shape means that one distribution is skewed relative to the other. An "S" shape implies that one distribution has longer tails than the other. In the default configuration a plot from qqnorm that is bent down on the left and bent up on the right means that the data have longer tails than the Gaussian.
References
Chambers, J. M., Cleveland, W. S., Kleiner, B. and Tukey, P. A. (1983). Graphical Methods for Data Analysis. Wadsworth, Belmont, California.
Hoaglin, D. C., Mosteller, F. and Tukey, J. W., editors (1983). Understanding Robust and Exploratory Data Analysis. Wiley, New York.
See Also
Normal, qq, qqline,
Examples

    set.seed(493)
    my.sample <- rt(100, 5)
    lab <- "100 observations from a t-distribution with 5 df"
    qqnorm(my.sample, main = lab, sub = "QQ Plot with Line")
    qqline(my.sample)

Package stats version 6.1.5-18
Package Index