Let's say you've got this data, with this line chart. The default is to leave the missing data out completely.
If you don't want that, though, you can double-click the chart, then click on one of the data lines, and choose Object Properties.
Select another option, such as the one setting all missing values to 0.
That might or might not be what you want.
The Continue Line option gives the best-looking, if maybe not the most accurate, results.