The theme actually isn’t part of the Highcharts namespace internally, it is just assigned there for conveniency.
Best practice currently would be to merge in the theme with your chart options:
chart1 = new Highcharts.Chart(Highcharts.merge(options1, theme1));
Black Label kindly shared their grouped X-axis plugin: https://github.com/blacklabel/grouped_categoriesSteve Leigh commented
If it helps, I've added some more detail on how this could be defined, and a hacked up example rendering in highcharts: http://jsfiddle.net/pMA2H/1/Steve Leigh supported this idea ·