Hi,
Thanks for contacting us with your question!
You can achieve this by setting
tooltip.valueDecimals to fixed number, this way it will only display decimals up to a certain number.
Demo: https://jsfiddle.net/BlackLabel/r9mxj7p2/
API: https://api.highcharts.com/highstock/to ... ueDecimals
Let me know if this is what you were looking for.
Best regards!