Layout Templates and matplotlib figures

Start by reproducing the Layout Templates example in the issue in Jupyter Notebook and JupyterLab, then read the discussion in #117 and the existing work in #174. The desired outcome is not yet defined; clarify an approach that lets matplotlib figures fill the widget layout while preserving their aspect ratio, then establish how the behavior should be verified.

see #117 (issue) and #174 (PR) for some discussion on the topic.

@martinRenou I have been playing with using Layout Templates to stack figures in fun ways. Unfortunately, since matplotlib is currently responsible for the figure size, I end up with situations such as the following, where the figures are all cut off a bit. This is particularly noticable in jupyter notebook since the screen width is reduced.

In the following example, I stack a 4x4 of buttons and then a 4x4 of figures. The buttons stretch nicely, but the figures do not.

%matplotlib widget
import matplotlib.pyplot as plt
from ipywidgets import TwoByTwoLayout
from ipywidgets.widgets import Button, Layout

# Buttons
def button(name):
    return Button(description = name, layout=Layout(height='auto', width='auto'))

a,b,c,d =  button("1"), button("2"), button("3"), button("4"),
layout_2x2 = TwoByTwoLayout(top_left=a, 
                            top_right=b,
                            bottom_left=c,
                            bottom_right=d)
display(layout_2x2)

# Figures
figs = []
plt.ioff()
for i in range(4):
    fig, ax = plt.subplots()
    ax.imshow([[1,2], [3,4]])
    figs.append(fig.canvas)

a,b,c,d = figs

layout_2x2 = TwoByTwoLayout(top_left=a, 
                            top_right=b,
                            bottom_left=c,
                            bottom_right=d)
display(layout_2x2)

In jupyter lab the problem is less so, but still unfortunate that figures basically ignore the flexbox layout the widgets are designed with. I would set the size with matplotlib, but since I don't know the size of the viewport the user is using, it's likely that the size I choose will be too small or too large depending on the user.

I don't have a concrete solution at hand, but I'm wondering if we can come up with some logic that preserves aspect ratio but otherwise fills a widget to its fullest extent?