init = function(data, seq){ 
            // Linear Scale: Data Space -> Screen Space; 
            xScale = d3.scaleLinear()
            .domain([0, dates.length-1])
            .range([0, innerWidth]);

            // Introducing y-Scale; 
            yScale = d3.scaleLinear()
            .domain([up_max, 0, -low_max]) // "extent" is equivalent to [d3.min(data, xValue), d3.max(data, xValue)]; 
            .range([innerHeight, innerHeight / 2, 0])
            .nice();

            nyScale = d3.scaleLinear()