Extra Items in Legend in Numbers
RE: Extra Items in Legend in Numbers - Apple Community
I had a nearly identical problem: an extra unlabeled symbol in a chart legend with no corresponding series displayed on the chart.
This was remedied in my case by first deleting 3 empty hidden columns in the sheet containing the chart data (which were present in the sheet I copied from, & were now not needed, one of which previously contained data I had included in another chart). The legend symbol now had a label corresponding to the header of one of the remaining columns. Then I edited the chart references to delete that particular reference and the unwanted legend item disappeared.
MacBook Pro (15-inch, 2018), macOS 10.14.6, Numbers 6.2.1