Allow to combine positive and negative values in stacked bars charts

As noted here: https://gitlab.com/gitlab-org/quality/team-tasks/issues/129#note_171940907

I think we might want to add a closed and created rate of issues in general and not just scoped to ~bug.

Ideally can we just use a chart to combine both ? E.g. https://www.chartjs.org/samples/latest/charts/bar/stacked.html