Multiple xs with JSON?

I think this might relate to #839 but I'm not actually trying to load multiple jsons, just one with multiple xs. I just didn't find a way to define multiple xs when using JSON, tried to use the same syntax as in the examples:

http://jsfiddle.net/ranzq/8oqt10sh/

I had a problem with a combination chart like this with single x. If a json point didn't have a value for the _step chart_, it was cut off and continued from next valid point. Multiple xs seemed to fix this but changing away from json format wouldn't be a nice option.