fix: tooltip pointer position in hovermode x

[OBe95]

Attempt to fix https://github.com/plotly/plotly.js/issues/6900

If I understood well the logic in hover.js when hovermode === 'x'

• lx represents the center of the highlighted data point
• there is a logic to adapt the tooltip box so that it doesn't go outside of the main-svg
• this logic seems to misplace the center of the tooltip pointer