The goal here is to avoid having to hard code what visualizers are shown
from the .ui files. We just should add things that we find are supported.
Long term, I think we'll add a concept of an "instrument" which is the
combination of a data source (SpSource) and a visualizer (SpVisualizerRow).
The other goal here is to enable the future support for discovering marks
and adding rows for each of those threads.
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