Describe the bug
The tooltip label the x value as X Value if not title on horizontal axis is available.
If multiple horizontal axis are available and only one axis title configured, the tooltip will shows X Value also if we have titles on it.

To Reproduce
Steps to reproduce the behavior:

1. Go to Area Chart - with 4 axes storybook story (remove a title from one of the two horizontal axes)
2. Hover over any data point

Expected behavior
Always display at least one title independently of the status of the axis titles.

We also have to include the possibility to add an hidden title on an axis so we can have the label on the tooltip but no axis title displayed on the chart

Screenshots

Version (please complete the following information):

• OS: osx
• Browser: chrome
• Elastic Charts: > 2.0.0

Additional context
n/a

Errors in browser console
n/a

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