Fix draw_tree

[dawe]

There are some issues with draw_tree, that is

• it doesn't take into account continuous values outside [0, 1] range (could be fixed rescaling data)
• it raises some warnings due to recent updates in matplotlib

MatplotlibDeprecationWarning: 
The modification of the Axes.artists property was deprecated in Matplotlib 3.5 and will be removed two minor releases later. Use Axes.add_artist instead.
  self.insert(len(self), value)

• it doesn't plot in notebooks, again due to changes in matplotlib