Plot a picture of a tree.
p is a vector of parent pointers, with p(i) == 0 for a root.
c is a color and character for nodes, or '' to not plot nodes.
d is a color and character for edges, or '' to not plot edges.
c or d may be omitted, and reasonable defaults are used.
See also ETREE4VHSWQ, TREELAYOUT , ETREEPLOT .