向D3散点图添加y轴水平线

var svg = d3.select("#chartContainer")
            .append("svg")
            .attr("width", width + margin.left + margin.right)
            .attr("height", height + margin.top + margin.bottom)
            .append("g")
            .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

var line = svg.append("path")
              .datum([{x: 0, y: height / 2}, {x: width, y: height / 2}])
              .attr("class", "line")
              .attr("d", d3.line()
                           .x(function(d) { return d.x; })
                           .y(function(d) { return d.y; }));

.line {
  fill: none;
  stroke: #ccc;
  stroke-width: 1;
}