Hi Alex,
As of now, we don’t have support to customize the rangenavigator tooltip format in stockchart. So, we have considered your requirement as improvement and logged a feature request on this. Based on other logged tasks priority, we will include this improvement in any of upcoming release. You can keep track of an improvement from the feedback portal below.
If you have any more specification/precise replication procedure or a scenario to be tested, you can add it as a comment in the portal.
Regards,
Durga G