plot_gbm plots the fitted values against the real values

gbm_plot(x)

## Arguments

x
a gbm.step object to be plotted

A ggplot2 plot

## Details

This is a development function that aims to improve the default plots used in gbm.step by elith et al. Their plots are good, but I want them to be better. In the future, this might even be implemented in ggvis. NB: This currently only works for gbm.step functions. In the future more features will be added so that it works for many decision trees.