Calculate available arc height "levels".
        Used to calculate arrow heights dynamically and without wasting space.

        args (list): Individual arcs and their start, end, direction and label.
        RETURNS (list): Arc levels sorted from lowest to highest.

def get_levels(self, arcs):
        """Calculate available arc height "levels".
        Used to calculate arrow heights dynamically and without wasting space.

        args (list): Individual arcs and their start, end, direction and label.
        RETURNS (list): Arc levels sorted from lowest to highest.
        """
        levels = set(map(lambda arc: arc["end"] - arc["start"], arcs))
        return sorted(list(levels))