Show user how to use Agg buffer in example

[darkdragon-001]

Tell the user to use Agg via

import matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt

matplotlib/examples/misc/agg_buffer_to_array.py

Line 8 in 507bd38

import matplotlib.pyplot as plt

---

An alternative is to explicitly create an Agg canvas via

from matplotlib.backends.backend_agg import FigureCanvasAgg
canvas = FigureCanvasAgg(fig)

as shown in this example

matplotlib/examples/user_interfaces/canvasagg.py

Line 30 in 507bd38

canvas = FigureCanvasAgg(fig)

[tacaswell]

@darkdragon-001 Does the change in #14797 address your concern?

[darkdragon-001]

@tacaswell Thanks for your fast response.

Unfortunately, it doesn't work:

AttributeError                            Traceback (most recent call last)
<ipython-input-1-5791127bf4b1> in <module>()
      9 ax.plot([1, 2, 3])
     10 ax.set_title('a simple figure')
---> 11 fig.canvas.draw()
     12 
     13 # grab the pixel buffer and dump it into a numpy array

AttributeError: 'NoneType' object has no attribute 'draw'