为负值修改amcharts中背景颜色的位置

<script src="https://cdn.amcharts.com/lib/4/core.js"></script>
<script src="https://cdn.amcharts.com/lib/4/charts.js"></script>
<script src="https://cdn.amcharts.com/lib/4/themes/animated.js"></script>

<div id="chartdiv" style="width: 100%; height: 500px;"></div>

// 创建图表对象
var chart = am4core.create("chartdiv", am4charts.XYChart);

// 设置数据
chart.data = [{
  "category": "Category 1",
  "value": 10
}, {
  "category": "Category 2",
  "value": 20
}, {
  "category": "Category 3",
  "value": 15
}];

// 创建类别轴
var categoryAxis = chart.xAxes.push(new am4charts.CategoryAxis());
categoryAxis.dataFields.category = "category";

// 创建值轴
var valueAxis = chart.yAxes.push(new am4charts.ValueAxis());

// 创建柱状图系列
var series = chart.series.push(new am4charts.ColumnSeries());
series.dataFields.valueY = "value";
series.dataFields.categoryX = "category";

chart.theme = am4themes_animated;

chart.background.fill = am4core.color("#f1f1f1");