Line Chart: another couple of small fixes (#42047)

* add fromOpacity of linear gradient to lighten the stroke color

* ensure unqiue linear gradient id is unique so that it does not cause conflict if multiple charts exist

* changelog

* use better automated margin

* make margins more consistent

* add better x numTicks

* smallest x numTick set to 5

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/13532201211

Upstream-Ref: Automattic/jetpack@d663487

Author: kangzj