Remove items from chart _yField in EXTJS 5

Question

Hello all Here i have chart data like below

Data looks like on legndheading = ['g1','g2','g3','g4','g5','g6','g7','g8']

I want to remove last two items from legndheading and update to series[0]._yField

var legndheading = series[0]._yField;

legndheading = ['g1','g2','g3','g4','g5','g6','g7','g8']

remove last two items and append to series[0]._yField

i tried to remove but not working

series[0]._yField.RemoveAt(5);

But it's not working