SSRS - How to add stripline to chart

The stripline is in this example defined on X axis.

 

Remember: the value for StartWith is order of the axis value, not exact value (StartWidth):

=DatePart("ww", today(), FirstDayOfWeek.Monday, FirstWeekOfYear.System ) 
  -min(Fields!skp.Value)).

PS: Better variant is to use SSRS function CountDistinct() - solves problem with gaps in axis values:

=CountDistinct( IIF(Fields!skp.Value <= DatePart("ww", today(), 
FirstDayOfWeek.Monday, FirstWeekOfYear.System ), Fields!skp.Value, nothing) )