Instead of adding sorting expression to Graph's Sortings collection add it to Graph's CategoryGroups
Sortings collection. This will sort the columns according to the specified order.
Check the attached report demonstrating this approach (can be opened with the latest version of Standalone Designer
On the side note, you can also sort Graph's columns in interactive mode - check Sorting Graph
(Interactive Sorting Action) help article.
If the suggested approach does not help or your scenario is different please attach a sample report demonstrating the current settings of Graph item in a support ticket so we can provide you further suggestions.
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