Javascript 如何在d3.js中将网格线移动到条形图后面？

所以我们试图创建一个简单的条形图。正在玩轴游戏，想在条形图后面画网格线。绘制了条形图和网格线，但网格线位于条形图上方

JS：

var dataArray = [6,7,7.2,7.7,7.7,9.5,11.7,10.5,10.1];

var margin = {top: 80, right: 40, bottom: 0, left: 5};

var y = d3.scaleLinear()
          .domain([0,12])
          .range([300,0]);

var ticks = [0,2,4,6,8,10,12];
var gridlines = d3.axisLeft(y)
                  .tickValues(ticks)
                  .tickSize(-innerWidth);

var yAxis = d3.axisLeft(y)
              .tickValues(ticks)
              .tickSize(0);


var svg = d3.select("#barGraph1")
            .append("svg")
              .attr("height","100%")
              .attr("width","100%");

var barGroup = svg.append("g")
                        .attr("transform","translate("+margin.top+","+margin.left+")");
barGroup.selectAll("rect")
             .data(dataArray)
             .enter().append("rect")
                      .attr("height",function(d){return 300-y(d);})
                      .attr("width","35")
                      .attr("fill","red")
                      .attr("transform", "translate(30,0)")
                      .attr("x",function(d,i){ return 70*i;})
                      .attr("y",function(d){ return y(d);});

//gridlines
barGroup.append("g")
          .attr("class", "grid")
          .call(gridlines);

barGroup.append("g")
          .attr("class","axisY")
          .call(yAxis);

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在添加条之前添加网格线，它们将在背景中显示出来