The sparkline-chart component was built to support different color variants for the *hover*-markpoints:  For the first iteration, it was decided that the only color that was going to be added is `#000`. Going forward, we need to decide on what other colors should be supported, or if black is enough, to remove the variant-prop from the component. ### Relevant Issues / Discussions: * Sparkline Chart Issue: https://gitlab.com/gitlab-org/gitlab-ui/issues/366 * Discussion about the color variants: https://gitlab.com/gitlab-org/gitlab-ui/issues/366#note_228943572 * MR that adds the sparkline component: https://gitlab.com/gitlab-org/gitlab-ui/merge_requests/834