In [34]:

import matplotlib.pyplot as plt
plt.ion()

fig = plt.gcf()
plt.plot([1,3,2])

Out[34]:

[<matplotlib.lines.Line2D at 0x115a1c668>]

In [52]:

class MyFigure:
    width = None
    height = None
    obj = None
    def _repr_png_(self):
        metadata = {}
        if self.width:
            metadata['width'] = self.width
        if self.height:
            metadata['height'] = self.height
        pngdata = self.render_png()
        return pngdata, metadata
    
    def render_png(self):
        if isinstance(self.obj, int):
            pass #...
        else:
            data, metadata = get_ipython().display_formatter.format(self.obj)
            if 'image/png' in data:
                return data['image/png']

In [53]:

img = MyFigure()
img

/Users/minrk/dev/ip/ipython/IPython/core/formatters.py:345: FormatterWarning: image/png formatter returned invalid type <class 'tuple'> (expected (<class 'bytes'>, <class 'str'>)) for object: <__main__.MyFigure object at 0x117f99240>
  FormatterWarning

Out[53]:

<__main__.MyFigure at 0x117f99240>

In [55]:

img.obj = fig
img.width = 20

In [58]:

img.width = 100
img

Out[58]: