I would suggest using the date() function applied to the variable which you want to use as the x axis. You should apply this date() function within the LOAD script of your qlikview application, as like in the example below:
date(varName, MonthFormat) as formattedDate
You can then use formattedDate as a new variable within your charts. MonthFormat is actually taken from the SET statements at the beginning of your QlikView script.
I hope this helps. Please mark as answered if I have helped to answer your question.