You delete msg.parts.index and count so that will not work. Also msg._msgid is different for each message coming out of the splits, so you create 4 new msg.parts.id's
Still i am confused as to what the output should be.
If this is not it
[{"id":"91dc074a0491e77e","type":"change","z":"613df62afc8a16bf","name":"","rules":[{"t":"set","p":"payload","pt":"msg","to":"[{\"date\":\"2022-01-01T01:00:00.000Z\",\"y\":1.234},{\"date\":\"2022-01-01T02:00:00.000Z\",\"y\":2.456}]","tot":"json"},{"t":"set","p":"topic","pt":"msg","to":"y","tot":"str"}],"action":"","property":"","from":"","to":"","reg":false,"x":180,"y":900,"wires":[["7d11862c79e51848"]]},{"id":"3faa320b53878b27","type":"inject","z":"613df62afc8a16bf","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"x","payload":"[{\"date\":\"2022-01-01T01:00:00.000Z\",\"x\":1},{\"date\":\"2022-01-01T02:00:00.000Z\",\"x\":2}]","payloadType":"json","x":190,"y":960,"wires":[["91dc074a0491e77e","7d11862c79e51848"]]},{"id":"7d11862c79e51848","type":"join","z":"613df62afc8a16bf","name":"","mode":"custom","build":"object","property":"payload","propertyType":"msg","key":"topic","joiner":"\\n","joinerType":"str","useparts":false,"accumulate":false,"timeout":"","count":"2","reduceRight":false,"reduceExp":"","reduceInit":"","reduceInitType":"","reduceFixup":"","x":350,"y":920,"wires":[["562a6e9c39693d48"]]},{"id":"562a6e9c39693d48","type":"function","z":"613df62afc8a16bf","name":"function 2","func":"let x = msg.payload.x.sort((a,b) => a.date<b.date);\nlet y = msg.payload.y.sort((a,b) => a.date<b.date)\nmsg.payload = [];\nx.forEach((obj, index) => {\n msg.payload.push({\n date: obj.date,\n x: obj.x,\n y: y[index].y\n })\n})\n\n\nreturn msg;","outputs":1,"timeout":0,"noerr":0,"initialize":"","finalize":"","libs":[],"x":480,"y":920,"wires":[["cbd7adedd7be02e8"]]},{"id":"cbd7adedd7be02e8","type":"debug","z":"613df62afc8a16bf","name":"debug 34","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","statusVal":"","statusType":"auto","x":700,"y":880,"wires":[]}]
Can you supply the output you want at the two debugs.
Or low code way
[{"id":"3faa320b53878b27","type":"inject","z":"613df62afc8a16bf","name":"","props":[{"p":"payload"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"[{\"date\":\"2022-01-01T01:00:00.000Z\",\"x\":1},{\"date\":\"2022-01-01T02:00:00.000Z\",\"x\":2}]","payloadType":"json","x":150,"y":880,"wires":[["91dc074a0491e77e"]]},{"id":"91dc074a0491e77e","type":"change","z":"613df62afc8a16bf","name":"","rules":[{"t":"set","p":"y","pt":"msg","to":"[{\"date\":\"2022-01-01T01:00:00.000Z\",\"y\":1.234},{\"date\":\"2022-01-01T02:00:00.000Z\",\"y\":2.456}]","tot":"json"}],"action":"","property":"","from":"","to":"","reg":false,"x":280,"y":880,"wires":[["d71b1af2e8b0a6e5"]]},{"id":"d71b1af2e8b0a6e5","type":"split","z":"613df62afc8a16bf","name":"","splt":"\\n","spltType":"str","arraySplt":1,"arraySpltType":"len","stream":false,"addname":"","property":"payload","x":410,"y":880,"wires":[["3b86a059dfe90375"]]},{"id":"3b86a059dfe90375","type":"change","z":"613df62afc8a16bf","name":"","rules":[{"t":"set","p":"payload.y","pt":"msg","to":"y[msg.parts.index].y","tot":"msg"}],"action":"","property":"","from":"","to":"","reg":false,"x":310,"y":920,"wires":[["9acf99b03f145784"]]},{"id":"9acf99b03f145784","type":"join","z":"613df62afc8a16bf","name":"","mode":"auto","build":"object","property":"payload","propertyType":"msg","key":"topic","joiner":"\\n","joinerType":"str","useparts":false,"accumulate":false,"timeout":"","count":"2","reduceRight":false,"reduceExp":"","reduceInit":"","reduceInitType":"","reduceFixup":"","x":490,"y":920,"wires":[["cbd7adedd7be02e8"]]},{"id":"cbd7adedd7be02e8","type":"debug","z":"613df62afc8a16bf","name":"debug 34","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","statusVal":"","statusType":"auto","x":660,"y":920,"wires":[]}]
