Jean-Baptiste> I do not see the utilitiy of the b variable. Is it
    Jean-Baptiste> to keep some kind of scheme ? or just a left over
    Jean-Baptiste> from set_visible ?

It's a bug, just use

    def get_visible(self):
        "return the artist's visiblity"
        return self._visible

    Jean-Baptiste> By the way on which backend is the alpha channel
    Jean-Baptiste> supported

Agg, TkAgg, GTKAgg.

Since I know you are a GTK user, I suspect you'll be interested in
GTKAgg. It's identical to the GTK backend in terms of the widget set,
but the figures are renderer with agg.

I know you are interested in object_picker so I might as well deal
with that now :-). You have to make a couple of minor changes to to work with GTKAgg. In my apps I import
FigureCasvasGTK or FigureCanvasGTKAgg renamed as FigureCanvas, which
makes it easy to switch back and forth if you want.

import matplotlib

from matplotlib.backends.backend_gtk import NavigationToolbar, \
     error_msg, colorManager
from matplotlib.backends.backend_gtkagg import FigureCanvasGTKAgg as FigureCanvas

Then create your picker canvas like

class PickerCanvas(FigureCanvas):
    def button_press_event(self, widget, event):
        width = self.figure.bbox.x.interval()
        height = self.figure.bbox.y.interval()
        self.pick(event.x, height-event.y)

The rest is OK.