Echarts：自定义悬浮框的内容

本代码实现的样式是当标题长度大于20位的时候，让标题折行显示

formatter的回调参数params有时候是数组有时候是对象，需要自己打印来看

tooltip: {
    // " 是小圆点的样式
    formatter: function(params) {
                        const title = params[0].name;
                        let title1 = "";
                        let title2 = "";
                        let results = "";
                        if (title.length > 20) {
                            title1 = title.slice(0, 20);
                            title2 = title.slice(20);
                        } else {
                            title1 = title;
                        }
                        params.forEach(item => {
                            results +=
                                "" +
                                item["seriesName"] +
                                "：" +
                                item["value"] +
                                "
";
                        });
                        if (title2.length) {
                            return (
                                title1 + "
" + title2 + "
" + results
                            );
                        } else {
                            return title1 + "
" + results;
                        }
                    }
}