draw_chimera matplotlib deprecation warning

**Description**
draw_chimera fails with text error:
/home/jraymond/PYTHON3-VENV/OCEANSDK/lib/python3.6/site-packages/dwave_networkx-0.8.8-py3.6.egg/dwave_networkx/drawing/qubit_layout.py:195: MatplotlibDeprecationWarning: Adding an axes using the same arguments as a previous axes currently reuses the earlier instance. In a future version, a new instance will always be created and returned. Meanwhile, this warning can be suppressed, and the future behavior ensured, by passing a unique label to each axes instance.
ax = fig.add_axes([0.01, 0.01, 0.98, 0.98])

**To Reproduce**
import dwave_networkx as dnx
chimera_graph = dnx.chimera_graph(m=4)
dnx.draw_chimera(chimera_graph)

**Expected behavior**
Should draw graph and not print error

**Environment:**
- OS: [Ubuntu 16.04.4 LTS]
- Python version: [e.g. 3.6.9]
matplotlib 3.3.2
dwave_networkx 0.8.8

**Additional context**