Show data by week number

Hi Everyone,

I'm working on a line chart with some data calcul by week ,and i want the data not show  in format dd/mm/yyyy, but with the number of weeks in year. Like this chart, in 27 June 2016, i want it could show 'week 27'



I've try with the number of weeks, but it can't figure like a date format. Also i try to show the week number in tooltips, but it can't works with character data,  it's not bed if i could have the week number in tooltips.

Thank you in advance!