Fix warning from matplotlib

[zhuchcn]

Description

The matplotlib warning should be gone now.

Closes #742

Checklist

• This PR does NOT contain PHI or germline genetic data. A repo may need to be deleted if such data is uploaded. Disclosing PHI is a major problem.
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• The name of the branch is meaningful and well formatted following the standards, using [AD_username (or 5 letters of AD if AD is too long)]-[brief_description_of_branch].
• I have added the major changes included in this pull request to the CHANGELOG.md under the next release version or unreleased, and updated the date.
• All test cases passed locally.

[lydiayliu]

Excited to try this! But will the warning persist for users that don't use the docker?

[zhuchcn]

You shouldn't see this other than docker, because should have the write permission to the default matplotlib config directory.