Hi,
Thanks for contacting us with your question!
Range selector is strictly connected to
xAxis.minRange, which sets the minimum range than can be displayed. As documentation states:
The default minRange for the x axis is five times the smallest interval between any of the data points.
So with provided data the
minRange is bigger than one month, thus making "1m" button disabled. You can override this property by setting
minRange to your own time unit in miliseconds, e.g. days or month.
The problem with selecting range in date picker is a reported bug that appears on all Firefox browsers, you can track its status here:
https://github.com/highcharts/highcharts/issues/16556. If this problem appeared on other browser, please write your browser and OS name and version.
Demo: https://jsfiddle.net/BlackLabel/hur76xg1/
References:
https://api.highcharts.com/gantt/xAxis.minRange
Let me know if that was what you were looking for!
Best regards!