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 )

@maniac103 I seems I forgot to follow up on this PR. I just briefly checked the commit log of the app, but I have not found that this was resolved. Correct?

@holgerfriedrich Correct. The code still seems fine, but what's missing is testing it against a server that actually sends the interpolaton value. Do you have a 5.1 test setup at hand?

Thanks for the feedback. I will give it a try the next days then 👍 - I did not have a dev environment for Android until now.

No I have completed my setup.
Sitemap contains 2 charts, first linear, then step.

In basicui it looks fine,

The API explorer shows correctly:

{
  "name": "default",
  "label": "Telefon",
  "link": "https://home3:8443/rest/sitemaps/default",
  "homepage": {
    "id": "default",
    "title": "Telefon",
    "link": "https://home3:8443/rest/sitemaps/default/default",
    "leaf": false,
    "timeout": false,
    "widgets": [
      {
        "widgetId": "00",
        "type": "Frame",
        "visibility": true,
        "label": "",
        "labelSource": "NONE",
        "icon": "frame",
        "staticIcon": false,
        "unit": "",
        "mappings": [],
        "widgets": [
          {
            "widgetId": "0000",
            "type": "Chart",
            "visibility": true,
            "label": "Außentemperatur (inet) [4.8 °C]",
            "labelSource": "ITEM_LABEL",
            "icon": "temperature",
            "staticIcon": false,
            "pattern": "%.1f °C",
            "unit": "°C",
            "mappings": [],
            "period": "12h",
            "interpolation": "linear",
            "item": {
              "link": "https://home3:8443/rest/items/Wetter_Inet_Temp",
              "state": "4.75 °C",
              "stateDescription": {
                "pattern": "%.1f °C",
                "readOnly": true,
                "options": []
              },
              "lastState": "4.99 °C",
              "lastStateUpdate": 1766353842039,
              "lastStateChange": 1766353842039,
              "unitSymbol": "°C",
              "type": "Number:Temperature",
              "name": "Wetter_Inet_Temp",
              "label": "Außentemperatur (inet)",
              "category": "temperature",
              "tags": [
                "Temperature",
                "Measurement"
              ],
              "groupNames": [
                "gWetterDia"
              ]
            },
            "widgets": []
          },
          {
            "widgetId": "0001",
            "type": "Chart",
            "visibility": true,
            "label": "Außentemperatur (inet) [4.8 °C]",
            "labelSource": "ITEM_LABEL",
            "icon": "temperature",
            "staticIcon": false,
            "pattern": "%.1f °C",
            "unit": "°C",
            "mappings": [],
            "period": "12h",
            "interpolation": "step",
            "item": {
              "link": "https://home3:8443/rest/items/Wetter_Inet_Temp",
              "state": "4.75 °C",
              "stateDescription": {
                "pattern": "%.1f °C",
                "readOnly": true,
                "options": []
              },
              "lastState": "4.99 °C",
              "lastStateUpdate": 1766353842039,
              "lastStateChange": 1766353842039,
              "unitSymbol": "°C",
              "type": "Number:Temperature",
              "name": "Wetter_Inet_Temp",
              "label": "Außentemperatur (inet)",
              "category": "temperature",
              "tags": [
                "Temperature",
                "Measurement"
              ],
              "groupNames": [
                "gWetterDia"
              ]
            },
            "widgets": []
          },
...

The APP does not show correctly yet:
Let me check that this is really the PR build and not the one from main I had tested before....

If click the charts in the app, everything is shown stepped, regardless of the setting in the sitemap:

(blue color indicates my local change, note that only this chart is shown blue)

The current code in this PR expects LINEAR, not linear. Not sure whether the string sent by the server changed at some point?
Either way, I 'like' the inconsistency between label source and Interpolation values :-/

(blue color indicates my local change, note that only this chart is shown blue)

What do you mean by that? The locally rendered charts should use (roughly) the same colors as the server side rendered ones.

Yeah, got it!

Server side images:

client side, linear:

client side, step:

The only problem I see now is that for "undefined" the defaults differ (server side: linear, client side: step).

Will have a look later.

(btw: the blue color above was just to make sure that I have really built and deployed my code. I have removed it now.)

Ok, the default is now linear also for the client side rendering.

PR is ready for review 🎉