Word Meaning "A single thing that's an indicator of greater overall trends"?
Bellwether seems like an excellent usage for the situation you describe, because when you're talking about trends (at least, other than retrospectively) you're somewhat inherently talking about prediction. If prediction weren't relevant to the situation I might use exemplar or typifier, or perhaps something like quintessential example if I were wont to wax verbose.
Those are all for a single item; for a subset of items, representative sample comes thundering out of the land of statistics to dominate the lexical landscape.