Chart-xy should take explicit input for x axis

[filip-jezek]

e.g. valueref="0,2", like chart-time. This enables multiple XY charts. So far it supports only single valueref

[TomasKulhanek]

Don't understand chart-time do not take specific to ref to x axes - it takes time.
Chartxy takes first ref to x axis, second ref to y axis. Mockup or picture may help. Note that chartjs https://www.chartjs.org 2.9.4 is used. There is a plan to upgrade chartjs 3.x, however, some refactor is needed.