By "draw free line in the line chart" I guess you mean "present a virtual series like a target line or trend line"?
Just as you prepare and send database data to a chart you can mix in any other series of virtual/computed data you desire.
If you truly want to draw non series type annotations then you need to import a lib like eCharts which has a very rich API and D.I.Y it
There is an example flow on the forum for importing and using eCharts. To manipulate it further, you will need to read the eChart docs and pay with it in their playground. Here is one that renders a custom trend line: Examples - Apache ECharts
Here is a demo of eCharts in dashboard 2.0: How to configure Echarts in Dashboard 2? - #4 by Steve-Mcl
