Charts are one I've wondered about, do I need to try to describe the trend of the data, or provide several conclusions that a person seeing the chart might draw?

Just saying "It's a chart" doesn't feel like it'd be useful to someone who can't see the chart. But if the other text on the page talks about the chart, then maybe identifying it as the chart is enough?

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embedding-shape an hour ago | parent | next [-]

What are you trying to point out with your graph in general? Write that basically. Usually graphs are added for some purpose, and assuming it's not purposefully misleading, verbalizing the purpose usually works well.

	▲	freedomben 34 minutes ago \| parent [-]
		I might be an unusual case, but when I present graphs/charts it's not usually because I'm trying to point something out. It's usually a "here's some data, what conclusions do you draw from this?" and hopefully a discussion will follow. Example from recently: "Here is a recent survey of adults in the US and their religious identification, church attendance levels, self-reported "spirituality" level, etc. What do you think is happening?" Would love to hear a good example of alt text for something like that where the data isn't necessarily clear and I also don't want to do any interpreting of the data lest I influence the person's opinion.

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gostsamo an hour ago | parent | prev [-]

It depends on the context. What do you want to say? How much of it is said in the text? Can the content of the image be inferred from the text part? Even in the best scenario though, giving a summary of the image in the alt text / caption could be immensely useful and include the reader in your thought process.