There is no chart setting or "#n/a" in Numbers to make a line span over missing data points unless you hide those rows in the table. Charts, by default, ignore hidden data. That might not be what you want to do, though. I don't know if anyone has come up with a decent way span missing points with formulas (i.e., to "create" the missing points using a linear fit).
If it is a scatter chart, you can hide (filter out) the rows in the table that have the missing data and they will be ignored by the chart by default. The chart will connect all the other points. A quick filter where you deselect "blank" should do it.
If it is a category chart (such as a line chart) and you don't need to have the categories with missing data, you can do the same thing. If you need all the categories to be on the chart, even those without data, then I do not have a solution.
People often use the line chart when their X data is numeric and evenly spaced. A scatter chart can give the same result. If your data is like this, try a scatter chart and filter your table to hide the missing data.