A regular color legend can be boring. We can do it a little creatively.

Just opened the fresh Mary Meeker Internet Report 2019. The charts are evidently created using Tableau. I noticed that one of the dual axis charts (Silde #14) was hard for me to read. There is no color legend. For people who lack knowledge or context, it may take a while to figure out which is which. But it only takes a bit more effort to make it a lot more readable.
Here I used to the Superstore data to create a similar chart with alternative (not your regular) color legends. One is in the title. The other is in the axis. We can use either or both to give a clear annotation of colors.
Voila, hope this helps. Here is the workbook for download.