Yes, exactly, this is a new parameter to be appended to the URL.
I have done it similar to the yAxisDecimalPattern option, both described here in docs.
I thought I have added interpolation to all places, where yAxisDecimalPattern was used.

I don't know about then new client-side rendering. Maybe @maniac103 could give me some guidance.

It seems that the client side chart library doesn't support interpolation and it needs to be done manually before sending the values to the chart.

It seems that the client side chart library doesn't support interpolation

It does: there's LineDataSet.setMode(), which allows selecting LINEAR or STEPPED (the former being the default).
So when constructing the data set after loading from persistence API, we can use the value from the widget (which we have at hand there) to select the right mode.

@mueller-ma Should we split this up and fix the server side charts first?

@maniac103 I don't see how I could implement the client-side charts. Would you be willing to guide me, or could you take over the implementation? We still have a few showstoppers for the OH5.0 release to resolve.

@holgerfriedrich Yes, I can complete this PR, assuming I can push to your branch.

Thanks a lot. I added you as a collaborator.

@holgerfriedrich I added the client side rendering handling. It's untested for now though, as interpolation doesn't seem to be part of the sitemap response:

        {
            "widgetId": "030404",
            "type": "Chart",
            "visibility": true,
            "label": "Füllstand Zisterne (Liter) [5643 l]",
            "labelSource": "ITEM_LABEL",
            "icon": "cistern",
            "staticIcon": false,
            "pattern": "%d l",
            "unit": "",
            "mappings": [

            ],
            "refresh": 1800000,
            "service": "influxdb",
            "period": "3D",
            "item": {
                "link": "http://192.168.100.10:8080/rest/items/CisternFillingLiter",
                "state": "5643.00",
                "stateDescription": {
                    "minimum": 0.0,
                    "maximum": 1.0E+8,
                    "pattern": "%d l",
                    "readOnly": true,
                    "options": [

                    ]
                },
                "lastState": "5638.00",
                "lastStateUpdate": 1753096820370,
                "lastStateChange": 1753096820370,
                "type": "Number",
                "name": "CisternFillingLiter",
                "label": "Füllstand Zisterne (Liter)",
                "category": "cistern",
                "tags": [
                    "Measurement",
                    "Water"
                ],
                "groupNames": [
                    "CisternFillSensor",
                    "GroupPersistenceChartable"
                ]
            },
            "widgets": [

            ]
        }

This is with no explicit interpolation set. I tried to set interpolation (via UI), but Main UI doesn't save the page once I've done that.
I'm running 5.0 RC1. Am I missing something here?

(FWIW, openhab/openhab-core#4610 not making any changes to WidgetDTO seems suspicious to me - after all, that's what the sitemap JSON response is generated from. Also FYI @mherwege )