I encountered the same issue.
python: 3.8.3
backtrader: 1.9.76.123
matplotlib: 3.3.1
you can either downgrade to matplotlib 3.2.2 or refer this pull request to fix it yourself:
https://github.com/mementum/backtrader/pull/418
For code/output blocks: Use ``` (aka backtick or grave accent) in a single line before and after the block. See: http://commonmark.org/help/
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