Date ranges in heatmap chart not sorted properly

[RickyLam]

Hi all,

The date range of my heatmap chart somehow does not sort properly:

The date range in the heatmap chart above is from an expression of the dimension "Date Period": 

Date(WeekStart([OrderDate]), 'DD/MM/YY') & ' - ' & Date(WeekEnd([OrderDate]), 'DD/MM/YY')

I tried to wrap around the expression above by DUAL function like this:

Dual(Date(WeekStart([OrderDate]), 'DD/MM/YY') & ' - ' & Date(WeekEnd([OrderDate]), 'DD/MM/YY'), WeekStart([OrderDate]))

It does sort correctly; but the date ranges of the heatmap chart would become date serial number. Apparently the numeric value of the Dual function is displayed instead of the text value:

How can I display the date range text while internally using the WeekStart([OrderDate]) for sorting purpose? Please advise.