null should be preserved and treated as they are when not using scale - which is to create a gap in the line or terminate the line at the last non null point

var palette = new Rickshaw.Color.Palette();
var graph = new Rickshaw.Graph( {
	element: document.getElementById("chart"),
	renderer: 'line',
	interpolation: 'cardinal',
	series: [
		{
			data: [ { x: 0, y: 19 }, { x: 1, y: 30 }, { x: 2, y: 22 }, { x: 3, y: 29 }, { x: 4, y: 26 }, { x: 5, y: 35 } ],
			name: 'new york',
			color: palette.color(),
			scale:d3.scale.linear().domain([1, 50]).nice()
		}, {
			data: [ { x: 0, y: 49 }, { x: 1, y: 35 }, { x: 2, y: 32 }, { x: 3, y: 38 }, { x: 4, y: 37 } ],
			name: 'boston',
			color: palette.color(),
			scale:d3.scale.linear().domain([1, 50]).nice()
		}, {
			data: [ { x: 0, y: 19 }, { x: 1, y: 22 }, { x: 2, y: 10 }, { x: 4, y: null }, { x: 5, y: 17 } ],
			name: 'los angeles',
			color: palette.color(),
			scale:d3.scale.linear().domain([1, 50]).nice()
		}, {
			data: [ { x: 1, y: 9 }, { x: 2, y: 3 }, { x: 3, y: 50 }, { x: 4, y: null } ],
			name: 'san francisco',
			color: palette.color(),
			scale:d3.scale.linear().domain([1, 50]).nice()
		}
	]
} );
graph.render();
new Rickshaw.Graph.HoverDetail({ graph: graph });