Hello there,
Thank you for contacting us!
To achieve that, simply create another yAxis, set appropriate height and position on both axes, and then, link the second series to second yAxis.
Take a look at the
demo: https://jsfiddle.net/BlackLabel/9Lwo8ytv/
You can also have draggable panes, by importing the drag-panes module to your HTML.
Demo: https://jsfiddle.net/BlackLabel/ctx9mp85/
In case of any other questions, do not hesitate to ask,
Best regards!