Creates a custom BiPlot for visualizing the results of PCA
myBiplot(pc, colorfactor, scaling = 100, nudge = 1.2, N = 10,
dims = 1:2, ...)output of prcomp
a factor the same length as nrow(pc$x) to color the points
integer to scale the vectors showing loadings
numeric to offset labels for loadings
numeric vector of length 2 indicating which PCs to plot