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I think you remember wrong, however, it should be possible to do using our RenderContext2D API and a custom ChartModifierBase derived class.
We have an example which allows you to free draw a Line Series which I’ve uploaded to a Github repo here.
Take a look at that. This draws using a DataSeries however you could just as easily draw on Mouse Move and create multiple series for your free text. The text will zoom and pan as you zoom and pan the chart.
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