Chart element colours should match their resp. legend colour

[sophiamersmann]

Some chart types, like stacked bar charts, use a reduced opacity, like 80%, for their chart elements. The legend colours are rendered with 100% opacity, though, which means that the chart colours and legend colours don't exactly match.

Closely related to #3918

[marcelgerber]

Depending on the outcome of #3918, we might want to tweak this.

If we don't end up going with a 100% opacity as an outcome of that issue, then it is also very much an option to just make it so the legend also displays colors at 80% opacity to match the chart.