echarts tooltips formatting options are limited

When working on fine tuning the popover for the column and stacked column charts (relevant discussions here: gitlab-org/gitlab-ui!174 (comment 146172920), gitlab-org/gitlab-ui!183 (comment 146173625)) We realized that, for the desired behavior of the tooltip to be implemented. We would have to talk to the maintainers of echarts or contribute the wanted feature upstream ourselves.

The desired behavior being able to lock the popover into a particular data point and have it hide until you hover to another data point, that way when hovering the mouse over a "blank" space, you would only see the mouse and not an entry point that is currently not being selected/hovered. Also depending on the type of chart, in the case of column charts for example, we would like for the popover to lock in to the bar itself and not just the top of the bar.

Considering the challenges we're currently facing and some of the solutions available to us at the moment, it seems that the best course of action in the meantime is to use the same popover behavior as implemented with the area charts which basically follow the mouse position even if the data points are far away from each other

cc @ClemMakesApps @adrielsantiago and @svoegeli @ameliabauerly

Corresponding google doc: https://docs.google.com/document/d/1XcEFF6pnTQzLlKxIH_ugufbQyuVq0Tk-A_gJOsphEMQ/edit#heading=h.4yansgf2ghli (Internal for now as sharing permissions gets complicated otherwise)