[FIX] web: flickering in graph view

Chart.js did badly compute the graph canvas container height (width) by
using clientHeight (resp. clientWidth) that may be rounded up by the
browser. This leads to the appearence of a useless scrollbar in the
graph view and makes it flicker on some update.

closes odoo/odoo#84320

Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>