Plotting Points for QQplots
DESCRIPTION:
Returns a vector of probabilities suitable to produce a QQplot.
USAGE: 
ppoints(n, a=<<see below>>)
REQUIRED ARGUMENTS:
n:
sample size for which plotting points desired (if length(n)==1), or data against which the plot is to be made.
OPTIONAL ARGUMENTS:
a:
parameter that controls the precise placement of the plotting points, 0<=a<=1. The default is .5 if m>10 or .375 if m<=10. where m is n if length(n)==1 and length(n) otherwise.
VALUE:
the vector of probabilities, p, such that qdist(p) plotted against sort(y) gives a probability (QQ) plot of y against the distribution of which qdist is the quantile function. The result satisfies p[i]==(i-a)/(m+1-2*a).
DETAILS:
Returns a vector of probabilities, p, such that qdist(p) plotted against sort(y) gives a probability (QQ) plot of y against the distribution of which qdist is the quantile function.
REFERENCES:
Blom, G. (1958). Statistical Estimates and Transformed Beta Variables. Wiley, New York.
SEE ALSO:
qqplot .
EXAMPLES: 
plot(qlnorm(ppoints(y)), sort(y))   # log normal qqplot
plot(qlnorm(ppoints(length(y))), sort(y))   # the same thing


plot(qgamma(ppoints(rain.nyc2), 4), sort(rain.nyc2))