Hi Daniela,
If this is a categorical axis (e.g. you don't provide X values to the line data points then the values should be the catgorical index of the start / end points [0, 3] in the case of the first line. Otherwise if this is a DateTime scale should pass in date time values converted to double - using ToOADate():
double startValue = startDate.ToOADate();
Hope this helps - let us know if you meet any problems.
Best Regards,
Nevron Support Team