tree_plot: Visualise a Hierarchical Clustering Model
In DavidHofmeyr/PPCI: Projection Pursuit for Cluster Identification

Provides a visualisation of a hierarchical clustering model via two-dimensional projections of the data assigned to each node.

1	tree_plot(sol, labels, node.numbers, transparency)

`sol`	a clustering solution arising from one of the functions mcdc, mddc and ncutdc.
`labels`	(optional) a vector of class labels. if provided then points in different classes are plotted in different colours.
`node.numbers`	(optional) logical. if TRUE then numbers are added to the plot to indicate the order in which nodes were added to the model. if omitted then node.numbers = TRUE.
`transparency`	(optional) if ommitted then points in scatterplots are shown as solid. If set to a value in (0, 1) then points are shown with transparency. Lower values correspond with a greater degree of transparency.