Describe the bug
When we only have a single datum chart, the tooltip that appears on hover is positioned either to the left or the right of the element, pushing it outside of the chart.

To Reproduce

1. create single datum chart
2. hover over chart element
3. see tooltip overflowing chart

Expected behavior
Tooltip should be visible within the chart. For single datum charts, we probably want the tooltip to follow the cursor within the element.

Screenshots

Version (please complete the following information):

• OS: all
• Browser all
• Elastic Charts: 7.0.1

Additional context

Errors in browser console
none

Kibana Cross Issues
n/a

Checklist

• every related Kibana issue is listed under Kibana Cross Issues list
• kibana cross issue tag is associated to the issue if any kibana cross issue is present