Hi Chloé-Ann,
I am not sure I got the problem.
Are you trying to put 2 categories on the x axis? Are you trying to create with Knowage the chart you posted?
In that case you must put the 2 categories into the X axis box and enable the group by second category option.
Remember that if the group by option is enabled is not possible to add 2 measure so you can't have in the same chart both bars and lines displayed. Only one of them.
I hope I was helpful.
Regards,
Nunzia