I bet you have some selections (filters in NPrinting) applied which are causing data set to be filtered out. Since there is no data for the period where you have 0 value there is only 1 data point - hence no line.
check your filters and/or section access!
cheers Lech, When applicable please mark the correct/appropriate replies as "solution" (you can mark up to 3 "solutions". Please LIKE threads if the provided solution is helpful to the problem.