title: Colorbar is attached to an existing axis, instead of filling into it
body: <p>The problem is a bit hard to describe, but very easy to show. I create a grid with subplots on it, where the right column is filled by a tall subplot (approximately following <a href="https://matplotlib.org/stable/gallery/subplots_axes_and_figures/gridspec_and_subplots.html" rel="nofollow noreferrer">this</a>) which I want to use for the colourbar. Creating a new axis of a given size and using it for a colourbar is done in many code samples (see for example <a href="https://stackoverflow.com/a/43425119/10320024">here</a>), but it's not working for me.</p>
  <p>Here's an example with a plot layout the same as my real plot that reproduces the problem:</p>
  <pre><code>import matplotlib.pyplot as plt
  import matplotlib.colors as clt
  import numpy as np
  
  fig, axes = plt.subplots(3, 2, figsize=(15,8), tight_layout=True,
                           gridspec_kw={'width_ratios': [1, 0.02],
                                        'height_ratios': [2, 1, 1]})
  
  x, y = np.random.rand(500000), np.random.rand(500000)
  counts, xedges, yedges, im = axes[0, 0].hist2d(x, y, bins=(149, 336), norm=clt.LogNorm(), cmap='inferno_r')
  axes[1, 0].plot(np.random.rand(2184))
  axes[2, 0].plot(np.random.rand(2184))
  
  gs = axes[0, 1].get_gridspec()
  for ax in axes[:, 1]:
      ax.remove()
  axbig = fig.add_subplot(gs[0:, -1])
  bar = fig.colorbar(im, ax=axbig)
  
  axes[0, 0].set_ylabel(&quot;2D histogram&quot;)
  axes[1, 0].set_ylabel(&quot;unrelated data&quot;)
  axes[2, 0].set_ylabel(&quot;other unrelated&quot;)
  bar.set_label(&quot;colourbar&quot;)
  </code></pre>
  <p>(note that I use <code>add_subplot(gs[0:, -1])</code> to make the tall subplot, but something like <code>add_axes([0.8, 0.1, 0.03, 0.8])</code> has the same effect)</p>
  <p>And the output:
  <a href="https://i.stack.imgur.com/t9WtZ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/t9WtZ.png" alt="subplots" /></a></p>
  <p>Notice how the colourbar is added as a <em>tiny little</em> new axis, onto the existing axis which I created for it. I would expect it to fill in the existing axis, as in <a href="https://stackoverflow.com/a/43425119/10320024">this</a> or <a href="https://stackoverflow.com/questions/32462881/add-colorbar-to-existing-axis">this</a> example. What's going wrong? I'm running matplotlib 3.3.1 from inside spyder 5.0.0 with python 3.8.</p>