Histograms of Predictors at Tree Nodes
DESCRIPTION:
Computes and displays histograms for selected nodes of a fitted object of class "tree".
USAGE: 
hist.tree(tree, ..., nodes, screen.arg = <<see below>>,
          figs = c(1, length(vnames)), plot = T)
REQUIRED ARGUMENTS:
tree:
fitted model object of class "tree". This is assumed to be the result of some function that produces an object with the same components as that returned by the tree function.
...:
any number of variables to view. Each variable must be have a name from those in the names attribute of the data frame used in fitting tree.
OPTIONAL ARGUMENTS:
nodes=:
an integer containing the index (node number) of the node to be examined. If missing, user selects nodes interactively as described below.
screen.arg=:
integer denoting the screen to be used for the histogram plotting. By default the active current screen plus one is used. See split.screen and tree.screens for more details.
figs=:
the arrangement of sub-screens to further split the histogram screen for the plotting of the individual variable histograms. Generally, a vector of the form c(n,m) where n and m are integers.
plot=:
logical flag; if nodes is given, should the histogram be plotted?
VALUE:
an invisible (unless plot=FALSE) list with names from ... for the last node selected interactively or the one given in the argument nodes. Each component of this list is a list itself with two components with names x (the values going left) and y (the values going right).
SIDE EFFECTS:
For each node selected and for each of the variables in ..., a horizontal histogram of the observations going left is placed opposite to a histogram of the observations going to the right.
GRAPHICAL:
This function checks that the user is in split-screen mode. A dendrogram of tree is expected to be visible on the current active screen, and a graphics input device (for example, a mouse) is required. Clicking the selection button on a node results in the additional screens being filled with the information described above. This process may be repeated any number of times. Warnings result from selecting leaf nodes. Clicking the exit button will stop the display process and return the list described above for the last node selected. See split.screen for specific details on graphic input and split-screen mode.
NOTE:
You may need to attach the original data frame prior to using this function.
SEE ALSO:
split.screen , tree , tree.object , tree.screens .
EXAMPLES: 
z <- tree(Mileage~ Weight + Type, car.test.frame)
tree.screens()
plot(z) ; text(z)
hist.tree(z, Weight, Mileage) # click on node to produce histogram