Highcharts csv data series starting with a null va

问题:

I am trying to represent csv data with highcharts. I can not use a csv data that starts with a null value. How can I use a csv data where series starts with a null value. Though it works fine with 'empty' or '0' but I specifically need this to works with null or NA please check jsfiddle

My csv looks like this

Year,Annual mean,5 year mean
1880,-0.31,null
1881,-0.22,-0.25
1882,-0.28,-0.29
1883,-0.3,-0.29
1884,-0.33,-0.3

Thanks in advance