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The Report Chart Component Properties pane has an implicit Filter for the ID of the record from the Page that it is on. So it acts as a Filter, rather than a Parameter. (So I wonder what the performance implications are for very large reports - this report I'm building has a couple of thousand rows, but if you want to summarise a very large data set it may be slow).
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Also remember to give it a nice label, and check the Show Refresh button checkbox because our users have been conditioned to click Refresh before they trust any charts.
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