Describe the bug
x-axis line annotations for a single value scale in histogram mode are not correctly positioned, aligning with the leftmost tick along the x-axis.

To Reproduce
Steps to reproduce the behavior:

1. Create histogram series with only one data value with x value 0
2. Add line annotation at 0.5
3. See line annotation positioned at 0

Expected behavior
Should appear correctly aligned with data value's position along x-axis

Screenshots
Incorrect placement of 0.5 annotation when only 1 value (should be in center but is aligned with 0 tick:

Correct placement of 0.5 annotation when more than 1 value:

Version (please complete the following information):

• OS: all
• Browser: all
• Elastic Charts: 7.2.0

Additional context

Errors in browser console
none

Kibana Cross Issues

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