draw.t draws a t distribution with the specified df.  Optionally it draws in the t observed and the area corresponding to the p-value, or the critical value and the alpha level.  Optionally it draws the cumulative distribution function (cdf), but the default is to draw the probability density function.  The ncp argument controls whether the central or noncentral t distribution will be plotted.  

ARGUMENTS:

df is the degrees of freedom.

alpha = the alpha level for a test.  If alpha is specified, the critical value is plotted, and labeled.  If alpha is not specified, then the critical value is not plotted.  

Tobs is the observed value of t.  This is an optional parameter.  If it is used, the function plots the area corresponding to the pvalue as well as the t distribution.  

Xmax is an optional paramter.  Use it to control the maximum value on the X-axis. 

Ymax is an optional paramter.  Use it to control the maximum value on the Y-axis. 

tail = 1 if HA: mu < 0;  tail = 3 if HA: mu > 0;  tail = 2 if HA: mu  0.

ncp: If ncp = 0, then the draw.t function plots a central t distribution.  If abs(ncp) > 0, then draw.t plots a noncentral t distribution with noncentrality parameter ncp. WARNING:  R will not allow the user to set abs(ncp) > 37.62.  See ?dt for more information about this limitation. 

plot.dist: If plot.dist = TRUE (default), then a plot will be made.  If plot.dist = FALSE, then no plot will be made and instead a K x 2 matrix will be returned (quantiles vs density height). 

cdf:  If cdf = FALSE, then the plot or outputted coordinates are for the t density function.  If cde = TRUE, then the plot or outputted coordinates are for the cumulative distribution function of the t distribution.

type: An argument that is passed to the plot function.  The default, type = "l", specifies that lines will be plotted.  See ?plot for other choices.  

xlab, ylab: If xlab and/or ylab equals "default", then the default labels will be plotted on the X and/or Y axis, respectively.  There are different defaults for different choices of ncp and cdf.  The user can specify other axis labels.  

xlim, ylim:  If xlim and/or ylim equals NULL, then default values will be generated for xlim and ylim.  There are different defaults for different choices of ncp and cdf.  The user can override these defaults.