[FIX] chart: trend line tooltip for scatter/line charts For scatter/line chart whose labels were numbers, the trend line tooltip was showing something like `(15, 9)`. 9 was the correct Y value, but the 15 was referring to the fact that it was the 15th point in the generated trend line. Which made no sense to the user. This commit changes the tooltip to show only the Y value. closes odoo/o-spreadsheet#5268 Task: 4279972 Signed-off-by: Rémi Rahir (rar) <[email protected]>