You can just select unwanted marks and right click, there'll be option to exclude those marks from your view.
Option2: Check your measure values. If its holding values 0/<0 for that period then such drop will appear. If there is no zero values then I guess you've used some calculated measures and in else part you've assigned 0. To avoid such drop don't handle the nulls as zero, to do that exclude the else part from your calculation.
IF [Your Condition] THEN [Measure]
IF [Your Condition] THEN [Measure] END
Thanks man, option 2 works very well
Glad I could help