Improve docs on sending multiple series to the ui-chart node

### Description

The docs on how to send multiple series of data to the chart node in a single message simply say:

>If you want a single piece of data to plot multiple lines, you can set the Series property to JSON, and then provide an array of keys (e.g. ["key1", "key2"]), which will plot a data point for each key provided, from a single data point.

I've not been able to translate that into a working example - so we should add one to the docs.

### Epic/Story

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