charts: ability to refer to (persisted) items matching in time in expressions

[ewaldc]

The problem

A simple addon (that I developed) publishes 2 channels mapped to items:

• PVPC_Rate_Now (Number:Energy)
• PVPC_Is_Rate_Cheap_Now (Switch ON/OFF proxy for boolean)

It's currently impossible to create a simple chart that displays PVPC_Rate_Now with color 'green' when PVPC_Is_Rate_Cheap_Now is true/ON and color 'red' otherwise:

  series:
    - component: oh-time-series
      config:
        name: Rate
        gridIndex: 0
        xAxisIndex: 0
        yAxisIndex: 0
        type: bar
        item:  PVPC_Rate_Now
        color: '= items.PVPC_Is_Rate_Cheap_Now.state  === "ON" ? "Green" : "Red"'
        on: true

does not yield the desired result, since PVPC_Is_Rate_Cheap_Now refers to its current/last state rather than its matching state in time. See community topic

Your suggestion

Extend expressions to be able to refer to a historical item with matching timestamp for use in charts e.g.

color: '=  items.PVPC_Is_Rate_Cheap_Now.matchingState()=== "ON" ? "Green" : "Red"'

where matchingState picks up the timestamp from the current series timestamp/slot.

Your environment

Openhab 4.04 Linux ARM64

Additional info

Happy to contribute a PR, but would appreciate some guideance on what would be an acceptable approach and where to start. Unless of course there is another way to solve this problem.