It's been at least 15 years since you touched the Graph/Chart maker in Illustrator. OMG is it a pain and the auto generated legend/key is such a mess and it is impossible to edit.

Illustrator has a tool to create charts and graphs but it is cumbersome and not easy to use. Once you create a chart/graph it’s auto generated in grayscale and recoloring is difficult, the legend/key auto-created is also difficult to style or edit. Excel is better, a product that’s not even intended for graphic design is better at basic charts/graphs than a program built to illustrate things. Microsoft should not be eating your lunch like this, but they are.