Avro and Protocol Buffers are schema-description languages. Schema files (e.g. here) can be difficult to understand when there are lots of data structures and multiple files, so it is helpful to visualize the relationships between the data structures in a UML diagram. Example: here
Adam Novak originally wrote code to visualize Avro schema files: 1 and 2.
avro2uml builds on his original code to add in additional features like data clusters, clickable clusters, and some amount of automation.
protobuf2uml is the same idea, but using Protocol Buffers-described schemas instead of Avro.