d3 API zoom

常用的看明白了，还有几个地方不太明白。

zoom函数: area path circle

.on("zoom", function(){
                        that.svg_obj.select(".x.axis").call(xAxis);

                        var _lines = that.lines;
                        for(var i=0; i<_lines.length; i++){
                          that.svg_obj.select(".g"+i+" path.line")
                                      .attr("d", lineFunction(_lines[i].dataset))
                                      ;
                          that.svg_obj.select(".g"+i+" path.area")
                                      .attr("d", areaFunction(_lines[i].dataset))
                                      ;
                          that.svg_obj.selectAll(".g"+i+" circle").attr("d", function(){
                                  that.svg_obj.selectAll(".g"+i+" circle").attr('cx', function(d,i){
                                          return xScale(i);
                                         })
                                    .attr('cy', function(d){
                                      return yScale(d);
                                    })
                                         ;
                          });
                        }

                      });