0 fill null values with d3 series data

I'm working on a multi-line chart in D3 but am having some issues with the rendering. I'm trying to render two lines with data that looks like this: [ { key:"line 1", values: [ {x:1, y:1}, {x:2, y:2} ] }, { key:"line 2" , values: [ {x:1, y:1}, {x:2, y:2}, {x:3, y:3} ] } ] "line 2" renders just fine, but "line 1" stops rendering at x = 2. I understand that my dataset might be considered incomplete,