A neat little trick you may have seen in a recent visualisation of mine was to label the top line with more detail than the subsequent lines, allowing your users to gather more context without seeing more clutter.
The trick is extremely simple and can be done in about 10 seconds.
1. Create your chart
2. Create a calculated field containing an IF statement that makes use of the FIRST() function.
IF FIRST() = 0 then “label” end
3. Add this extra detail to the tooltip shelf and form the tooltip string as appropriate.
And voila you have your context labels (it is important to note that you may have to change the computed along method given your scenario).