Hello. I have a stacked-area chart. The X-axis is age and the Y-axis is the
number of hours spent on an activity. The data series represent various
groups of people. I would like to show one other variable - money spent on
the activity as it isn't directly correlated with the hours spent.

I've been envisioning some form of gradation in the series' area fills (like
heat maps or topography) but am unsure how to do that or if it's even
possible.

Any other ideas? I need to show variation within the stacked-area polygons.
I know that some people may advise to break this chart up into multiple
charts but I need a solution that has the entire solution on one graph or
map.

Thanks for any ideas or comments on what you would do.