For example, instead of one bar for each of 5 answers, show combined responses 1-4 and compare to response 5.
I don’t think you can accomplish this from the reporting end of things. It could be done by creating a recoded value in the survey. The best way to do a recode depends on the situation.
If you have already gathered all of your data, you could:
Create a copy of your question in the survey and set it to disabled.
Export your data
Use something like Excel and create a formula to recode the data from your question and store it in your new disabled question.
Import the data back in to your survey (see the Bulk Update Existing Answers on this page: https://help.surveygizmo.com/help/data-import?_ga=1.87004063.663635893.1466605465 )
Then report on the new copy instead of reporting on the original question.