Bug report

Bug summary

I've passed urls arg in the Axes.pcolormesh() method to set URLs in a QuadMesh, but when I save it to SVG the URLs are missing.

I traced it to the RendererBase.draw_quad_mesh() method which passes [None] as the urls argument to self.draw_path_collection().

Code for reproduction

import numpy as np
import matplotlib.pyplot as plt

nan = np.nan
data = np.array([[nan, 4, nan], [2, 7, 5], [nan, 8, nan]])
links = np.array([[nan, 'http://abc.com/', nan],
                  ['http://cnn.com/', 'http://nbc.com/', 'http://cbs.com/'],
                  [nan, 'http://nytimes.com',]])

fig = plt.figure()
ax = plt.gca()
mesh = ax.pcolormesh(data, urls=links)
fig.savefig('/tmp/test_links.svg')

Actual outcome

$ fgrep http /tmp/test_link.svg 
  "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<!-- Created with matplotlib (http://matplotlib.org/) -->
<svg height="288pt" version="1.1" viewBox="0 0 432 288" width="432pt" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">

Expected outcome

The URLs are inserted into the quadmesh element like this:

<g id="QuadMesh_1">
  <a xlink:href="proxy.php?url=https%3A%2F%2Fgithub.com%2Fmatplotlib%2Fmatplotlib%2Fissues%2Furl"> <path> /* the path */ </path> </a>
  <a xlink:href="proxy.php?url=https%3A%2F%2Fgithub.com%2Fmatplotlib%2Fmatplotlib%2Fissues%2Furl"> <path> /* the path */ </path> </a>
</g>

Matplotlib version

• Operating system: Linux
• Matplotlib version: 2.2.2
• Matplotlib backend (print(matplotlib.get_backend())): module://ipykernel.pylab.backend_inline, but SVG is what I care about
• Python version: 3.5.3
• Jupyter version (if applicable): 4.4.0
• Other libraries:

matplotlib is installed in a virtualenv with pip.