Legend/popover colour chip update for stacked column chart
Problem
From discussion here: gitlab-org/gitlab-services/design.gitlab.com#614 (comment 364661865)
- The color chip in the legend/popover doesn't really match the columns in charts
- It's hard to recognise that you can actually click chips in the legend to show/hide values in chart
- It's current hard to differentiate between lines and columns on the same chart: gitlab-org/gitlab-services/design.gitlab.com#615
Solution
Legend/popover colour chip update for stacked column chart
Style as described here: gitlab-org/gitlab-services/design.gitlab.com#617 (comment 370810027)