Heatmap title tooltip shows incorrect label below first panel

[nickofthyme]

Describe the issue

When using heatmap with SmallMultiples the tooltip axis label-only tooltip shows the incorrect label.

Expected behaviour

Tooltip show the actively hovered axes label

Screenshots

Additional context

The heatmap is the only chart that implements this axis label tooltip. Not sure if this is ideal UI because it is often duplicating the full label, maybe this could just show for custom axis label tooltips or truncated values.

Kibana Cross Issues

Related to elastic/kibana#154434