On stacked graphs (completed and failed), fill colours are merged

[nigeldeakin]

The fill colours used with stacked charts (i.e. completed and failed) do not match the fill colours shown in the legend.

See this screenshot:

In the "completed" graph, see how the legend shows the path /hello-cold-async-a2 as having a blue fill colour, but the actual fill colour used is a kind of green.

The green colour appears to be caused by the yellow fill extending from Y=30 all the way down to the X-axis, and being combined with the blue colour below Y=20.

This was reported in the following chart.js issues:
chartjs/Chart.js#2579
chartjs/Chart.js#2380

The second of these suggests a workaround. This should be applied. If it can't we should reconsider whether we should use a fill colour (and whether these graphs should be stacked at all).