the chart node needs a special input -> in this case it is an object.
to get 2 lines in your line diagram you have to transfer 2 objects to the chart node at the same time.
(see example)
[{"id":"a7976e20.a59f1","type":"function","z":"48461642.eb259","name":"","func":"msg.payload = [{\n\"series\": [\"A\", \"B\", \"C\"],\n\"data\": [\n [{ \"x\": 1504029632890, \"y\": 5 },\n { \"x\": 1504029636001, \"y\": 4 },\n { \"x\": 1504029638656, \"y\": 2 }\n ],\n [{ \"x\": 1504029633514, \"y\": 6 },\n { \"x\": 1504029636622, \"y\": 7 },\n { \"x\": 1504029639539, \"y\": 6 }\n ],\n [{ \"x\": 1504029634400, \"y\": 7 },\n { \"x\": 1504029637959, \"y\": 7 },\n { \"x\": 1504029640317, \"y\": 7 }\n ]\n],\n\"labels\": [\"\"]\n}]\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","x":260,"y":320,"wires":[["7d76732b.a947ac"]]},{"id":"1295c7f8.7e73c","type":"inject","z":"48461642.eb259","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"","payloadType":"date","x":110,"y":320,"wires":[["a7976e20.a59f1"]]},{"id":"7d76732b.a947ac","type":"ui_chart","z":"48461642.eb259","name":"","group":"c0b1b8ca.93ab38","order":2,"width":0,"height":0,"label":"chart","chartType":"line","legend":"false","xformat":"HH:mm:ss","interpolate":"linear","nodata":"","dot":false,"ymin":"","ymax":"","removeOlder":1,"removeOlderPoints":"","removeOlderUnit":"3600","cutout":0,"useOneColor":false,"useUTC":false,"colors":["#1f77b4","#aec7e8","#ff7f0e","#2ca02c","#98df8a","#d62728","#ff9896","#9467bd","#c5b0d5"],"outputs":1,"x":450,"y":320,"wires":[[]]},{"id":"c0b1b8ca.93ab38","type":"ui_group","name":"Default","tab":"50372f16.d4f1a","order":1,"disp":false,"width":"10","collapse":false},{"id":"50372f16.d4f1a","type":"ui_tab","name":"Home","icon":"dashboard","disabled":false,"hidden":false}]
The dashboard has a very good documentation so please see the link aswell
https://github.com/node-red/node-red-dashboard/blob/master/Charts.md
if you want to create a continuous diagram, so that the diagram retains the old values, this is not easily (to my knowledge) possible. I use plotly.js to do that.
What you can do in parallel is to save the previous data set via a context or flow variable, then add the new one and transfer everything to the chart node