Hi robsimpson!
Welcome to our forum and thanks for contacting us with your question!
You can do it a few ways. You can use
tooltip.pointFormat property and set a proper variable.
Demo: https://jsfiddle.net/BlackLabel/z2718Lah/
API Reference: https://api.highcharts.com/highcharts/t ... ointFormat
Or you can use
tooltip.formatter property and return a proper string. This is used to more complicated stuff or if you want to show some HTML element in the tooltip.
Demo: https://jsfiddle.net/BlackLabel/638yzLx5/
API Reference: https://api.highcharts.com/highcharts/tooltip.formatter
Feel free to ask any further questions.
Regards!