The Data Visualization Catalogue by Drew Skau.
From the post:
If you’ve ever struggled with what visualization to create to best show the data you have, The Data Visualization Catalogue might provide just the help you need.
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Severino Ribecca has begun the process of categorizing data visualizations based on what relationships and properties of data that they show. With 54 visualizations currently slated to be categorized, the catalog aims to be a comprehensive list of visualizations, searchable by what you want to show.
Just having a quick reference to the different visualization types is helpful by itself. The details make it even more helpful.