Helper function for the plot functions to add variable markers and labels to the predictor variable axes
make.dfs.for.X(Xo, P, B, xnames, mx, sdx)
output with information for the placement of variable markers and labels
Original predictor matrix
an integer indicating the number of predictor variables
matrix (P x S) with weights
a vector with variable names
averages of the original predictor matrix
standard deviations of the original predictor matrix