source selected 2d multiline

I am having trouble understanding how to select specific indices within a multiline source. I have a callback function that I have modified so that I can test the accuracy of the source[‘selected’] line only:

def update_line(source=source, window=None):

data =

slider_value = cb_obj.value



###call slider

slider = Slider(start=1, end=3000, value=1, step=1, callback=CustomJS.from_py_func(update_line))

This code correctly produces the data only for line # 3 on my map (it is a trackline). but it produces all of the data for line #3, and not solely points 0,1, and 2. Can anyone help me correct the source[‘selected’] call? Thank-you so much for any help!