setting hyper-parameters; 
        const svg = d3.select('#mainsvg');
        const width = +svg.attr('width');
        const height = +svg.attr('height');
        const margin = {top: 100, right: 220, bottom: 100, left: 120};
        const innerWidth = width - margin.left - margin.right;
        const innerHeight = height - margin.top - margin.bottom;
        const xValue = (datum) => { return dates.indexOf(datum['日期']); };
        const yValue = (datum) => {return datum['新增确诊']};
        //con