Hi Owain, this appears to be the natural way this graph is rendered, as it can be seen in the corresponding documentation for this type of graph, XY Area Chart - in this page.
You might be able to customize the display, but that would require some tweaking with the Graph component that should be living within the following location:
Hi team, I’m Avinoam, a product manager on Confluence Cloud, and today I’m really excited to let the Community know that all customers can now try out the new editing experience and see some of the ...
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