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tf is published in a distributed fashion, with multiple nodes publishing different parts of the tree to the /tf topic.

Practically, what this means is that each transform array that is published may or may not contain the transform information that you're looking for, and may or may not be in the same order each time. (it's also very difficult to plot the components of a quaternion and understand what is going on).

The transforms in the /tf topic only span a single link in the tree, so it isn't possible to directly extract a transform that spans multiple links. This is why the tf library exists.

I think the better approach here is your first suggestion; to write a node which subscribes to tf, looks up the transform that you're interested in, and republishes that for plotting.