Principal component analysis biplots - Understanding Biplots - page 118

"black" , size.centroid = 1 , pch.interp = 16 , col.interp = "red" , size
.interp = 1.2 , length = 0.15 , angle = 15 , lty = 1 , lwd = 1.75, ...
where the argument ... refers to any additional argument(s) passed to the function
polygon called by vector.sum.interp .

3.7.8 Function circle.projection.interactive

This function has the following arguments:

Specifies if a centre line is to be drawn. Default is
FALSE.

cent.line

Specifies colour of circle. Default is "black" . Colr

Specifies position of origin of biplot. Default is c(0,0) . Origin

... Optional arguments passed to the lines function controlling

appearance of circle.

The function circle.projection.interactive is called after a PCA biplot with
prediction axes has been created on a graph window. On calling this function the cursor
changes to a cross when moved over the biplot. Position the cursor at any point on the
biplot (usually one of the sample points). A circle is drawn with its centre the midpoint
of the line from the origin to the selected point. The intersections of the circle with the
prediction axes give the predictions on all variables for the chosen sample. This method
of prediction is called circular prediction and plays an important role in nonlinear biplots
(see Chapter 5).

3.7.9 Utility functions

The four functions draw.arrow , draw.rect , draw.text and draw.polygon are use-
ful for annotating any biplot and are called after a biplot has been drawn. They all
contain a ... argument for passing arguments to change the properties of the arrow,
rectangle, text or polygon to be created. After calling draw.arrow the two end posi-
tions for the arrow must be selected on the biplot; similarly, the bottom left and top
right position for the rectangle must be selected after calling draw.rect . The func-
tion draw.text has a required argument string specifying the string value to be
printed at the selected point on the biplot. Printing in the figure margins is allowed and
control over the exact printing position is managed through the ... argument. In sev-
eral of the biplots shown in this topic we have made use of draw.text to manually
adjust some of the axes labels for improved legibility. The draw.polygon function has
a required integer-valued argument vertex.points specifying the number of vertex
points needed. The default vertex.points = 3 results in a triangle being drawn after
selecting the three positions for the vertices on the biplot. The function draw.polygon
returns the coordinates of the vertices as well as the coordinates of the centroid of the
drawn polygon.