未捕获的类型错误:无法读取d3中未定义的属性“长度”。

I am trying to make a line chart in d3, with time on x-axis. I am using json data in variable. I used d3.time.format() function to format the time, but it gives me above error. I am learning d3 so please help. my code is :I am trying to make a line chart in d3, with ti