fix matplotlib compatibility

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 84.04%. Comparing base (e8ddfd7) to head (9a4e859).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #384 +/- ## ======================================= Coverage 84.04% 84.04% ======================================= Files 52 52 Lines 9141 9141 ======================================= Hits 7683 7683 Misses 1458 1458 ``` | [Flag](https://app.codecov.io/gh/zktuong/dandelion/pull/384/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Zewen+Kelvin+Tuong) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/zktuong/dandelion/pull/384/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Zewen+Kelvin+Tuong) | `84.04% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Zewen+Kelvin+Tuong#carryforward-flags-in-the-pull-request-comment) to find out more.

[zktuong]

closes #383