In the Line, bar, pie template, you can set series info popups. This allows to write custom text, which will only appear in the popup hovering over the specified series.
To do this first go to the popups and panels section. Next, toggle "Set series info for popups".
In the text box you can add info for each series by typing "series::info you want to add". To add more than one series, just start a new line.For example, in the gif below I've added comments about both of the series by adding this text:
Low-carbon sources :: Includes wind, solar, hydro, biofuels, other renewables and nuclear
Fossil fuels sources :: Includes oil, gas and coal
If you hover over the series, the information added will now appear in the popup
Note: setting series info in popups only works when custom popups are not used.