Numbers: epxanding a range of a chart

OK, I know the (to my mind unituitive but there you go) way of exapnding a graph's range. You click on the graph, a blue tab appears above the range, you command-click it and a white circle you can drag then appears at the bottom-right of the range. Drag the circle, and you're done.

Fine, if slightly nuts. However, I have no idea what a -red- tab means when you click a range. This red tab isn't responding to command click, so I don't know how to expand the range of one of my graphs. I have seven graphs in this sheet so don't really want to do them all again, besides which the other six all responded fine.

Any ideas?

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