Feature Request: Exclude missing nodes

[jonfitt]

I have a repository of protobuf files that use some basic types from another repo which is not available at the time when I am generating graphs. StringUtf8Value, BoolValue, ResourceName, that kind of thing. I really don't need them graphed.

I would like to exclude the diagramming of missing nodes as it clutters the output just pointing to a Node that says "this type is missing"

Thanks for your consideration, I love this tool it really helps to map things out in my mind.