What do you think of using this instead in __init__

if self.__class__ == FigureCanvasX:
  warn.deprecated(...)

It feels safer and cleaner to me, checking as early as possible and ensures we warn only if someone trys to create an instance of this base class.

Fine with me. I will get back to this as soon as I have a chance

Cool, do we still want this in for 2.0?

Yes thats my intention. We either need to do this or fix the GDK/GTK backend which is broken. Like this we will at least warn users about this and encurage them to use GTKAgg instead.

I don't know enough about the graphics plotting to know why someone would not want to use Agg or Cairo.

I don't think many users will but it you have to select a GTK backend and don't read the docs carefully it's very easy to select the GTK backend over the GTKAgg since it's just

import matplotlib
matploltlib.use('GTK')

Sure, I have done that, I didn't realise that a difference existed between Gtk and GtkAgg. Gtk feels like it should alias GtkAgg, i.e. due to most systems not installing cairo by default. As a simple user I wouldn't want to mess around figuring out whether I should use Agg or Cairo. Maybe something for a Getting Started guide that puts it real simple why users would choose one over the other.