Is there a particular reason to specify offset:0 and offset:1 or could I drop the first and last line altogether, like so? .selectAll("stop") .data([ // {offset: 0, color: "red"}, {offset: y(threshold) / height, color: "red"}, {offset: y(threshold) / height, color: "black"}, // {offset: 1, color: "black"} ])

Apparently those lines were unnecessary in this example, so, removed. Thanks.