I had the same question this year (2021), and it looks like there’s support for it now. In short, you just have to double-click on the empty legend item for the series, and it’ll give you a place to put text, directly in the chart.
Here’s what my chart looked like (with no series titles):
If you double-click on one, you’ll get a small text input area.
Type in your series name and then press enter or click outside of the input area.
Repeat this for each series, and you should be good to go.
