Edit chart error: "The group expression must not be empty."

cjmaloof

Edit: solved, newbie error. Didn't realize I needed to "group" data with only one raw data point per chart data point, and was confused by where the error was showing up.

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"The group expression must not be empty."

Can anyone explain what this error means and how to fix it? (Or what other information would help in identifying it?) I'm editing my first bar chart; it seems straightforward, but this error appears once I've selected the X and Y series, and Google knows nothing.

The "Optional Y Series Grouping" box is empty, but putting something in it doesn't help, and I'm afraid I can't see what other "group expression" the error might be referring to.

Thanks for any advice...