XYChart: Support for regression lines

### Discussed in https://github.com/grafana/grafana/discussions/63173 <div type='discussions-op-text'> ^{Originally posted by **hafeja** February 9, 2023} Hey! As user of plugin [XY Chart](https://grafana.com/grafana/plugins/xychart/) I'd like to request support for **regression lines** for this plugin. Finally, a regression line should look e.g. like this:  **Expected Benefit** A regression line transports the key message of a scatter plot to dashboard viewers **at a glance**. It would be good to support different kinds of regression lines (Linear, Exponential, Power, ...) similar to the ones provided by the plugin [Scatter](https://grafana.com/grafana/plugins/michaeldmoore-scatter-panel/):   The **icing on the cake** would be the possibility for the dashboard creator to define the formula that is used to calculate the regression line. </div>