Problem with Line Chart and the same values in the same date

system_migrated

Hi, I have data as below:

Date Value
2010-06-15 20
2010-06-15 60
2010-06-16 60
2010-06-16 20
2010-06-20 20

And I have problem with Line Chart I would like chart look like second in image because now is like first. Someone know how can I do this ?