Have you considered supported a ‘show line’ option with area charts? I found a workaround, but it won’t work for stacked area charts.
For a non stacked chart, i’m just creating the same data set twice, once as ‘line’ and again as ‘area’. But I can’t do this with stacked area chart because there is no ‘stacked line’ type, only ‘stacked area’
See this image, it really only makes sense when using transparency for the area chart – but adds a really nice aesthetic.