Hey Yves Le Cochec,
I'm pretty new to Tableau Desktop so this took me a bit to figure out but I'm attaching a workbook which shows Profit vs. Sales in 2012 and 2013, with a line connecting the dates. In this particular case, I had to drag the dates to the "details" tooltip and change the marks to "Line" instead of "Automatic." Then I threw dates on the label tooltip to show each year. After that I right-clicked dates and filtered down to the dates I wanted.
Let me know if you have any questions!
connecteddates.twbx 1.4 MB