You’re right, the “Extra” label is missing. It’s missing on maps as well. This might need to be reported as an issue.
I have done your work around in the past so that, not only do I get the data to show in the tooltip, but it also shows in the data table that’s below the chart.
Just uncheck “Visible” in your series. You may also need to turn off the different states (hover, select, etc) too. I’ve had to make sure that my colours were set to transparent so they wouldn’t show.