Fix broken doc build due to Matplotlib 3.0 release

[hcho3]

No description provided.

[codecov-io]

Codecov Report

Merging #50 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #50   +/-   ##
=========================================
  Coverage     85.39%   85.39%           
  Complexity       35       35           
=========================================
  Files            76       76           
  Lines          5121     5121           
  Branches         21       21           
=========================================
  Hits           4373     4373           
  Misses          727      727           
  Partials         21       21

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 25f4a17...2847029. Read the comment docs.