Describe the bug
An extra line appears on an area chart.
The issue reproducible in the storybook

Expected behavior
The issue wasn't reproducible in 3.4.4 and earlier

Screenshots
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Version (please complete the following information):

• Elastic Charts: 3.4.5

Additional context
Add any other context about the problem here.

Errors in browser console
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Kibana Cross Issues
Add any Kibana related issues here.

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