find reducer #1914
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find reducer #1914
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* find transform * find reducer * prefilter data * extent * find documentation --------- Co-authored-by: Philippe Rivière <fil@rezo.net>
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Broken out of #1896, this introduces the find reducer which works with the group (or bin) transform to pull out a specific value for each group. This allows the group transform to function as a “pivot wider” transform; for example, it can take “tall” data with a row for each male and female observation and output “wide” data with a column each for male and female observations. This is probably most useful for the difference mark where you want to compare two metrics, but we demonstrate it here using the arrow mark.
To make it easier to implement the find reducer, this PR also enhances the extent supplied to reducers by the bin and group transforms: the extent now includes the (possibly transformed) data. And in the case of the group transform, the extent now includes the group’s x or y value as appropriate.
TODO
Given how close this is to the group transform, I wonder if there is a way to do this without needing to invent a new transform. Like, could there be shorthand for a “find” reducer? Then it could work with the bin transform, too, for example.Done.