'Makes all streamlines readable as a single trace.
:rtype (list, list): streamline_x: all x values for each streamline
combined into single list and streamline_y: all y values for each
streamline combined into single list'

def sum_streamlines(self):

    streamline_x = sum(self.st_x, [])
    streamline_y = sum(self.st_y, [])
    return (streamline_x, streamline_y)
