The function borrow_secondary function does not return a mutable reference. It would be useful if it actually would be possible to get a &mut reference to the secondary context.
Implement a new function that actually return a &mut reference to the secondary context.

For example to write a generic draw function:

 fn draw_chart_series<DB>(
 chart: &mut ChartContext<'_, DB, Cartesian2d<RangedCoordf32, RangedCoordf32>>,
 data_series: &ModelRc<XYData>,
 ) where
 DB: DrawingBackend,
 {
 // draw the chart series for both primary and secondary
 }