By default, the bar is placed in a corner of the graph depending on
the direction of the tree. Otherwise both x and y must
be specified (if only one is given it is ignored).
The further arguments (...) are used to format the text. They
may be font, cex, col, and so on (see examples
below, and the help page on text).
The function locator may be used to
determine the x and y arguments.