Echarts 柱状图、折线图等，x轴内容过长时换行显示

需修改 xAxis.axisLabel.formatter

代码如下：

formatter: function(value)
{
    var str = ""; 
    var num = 4; //每行显示字数 
    var valLength = value.length; //该项x轴字数  
    var rowNum = Math.ceil(valLength / num); // 行数  
    
    if(rowNum > 1)
    {
        for(var i = 0; i < rowNum; i++)
        {
            var temp = "";
            var start = i * num;
            var end = start + num;
            
            temp = value.substring(start, end) + "\n";
            str += temp; 
        }
        
        return str;
    }
    else
    {
        return value;
    } 
}