text: // render error bars
  function renderErrorBar(params, api) {
    var xValue = api.value(0);
    var highPoint = api.coord([xValue, api.value(1)]);
    var lowPoint = api.coord([xValue, api.value(2)]);
    var halfWidth = api.size([1, 0])[0] * 0.1;
    var style = api.style({
        stroke: api.visual('color'),
        fill: null
    });
  
    return {
        type: 'group',
        children: [{
            type: 'line',
            shape: {
                x1: highPoint[0] - halfWidth, y1: highPoint[1],
                x2: highPoint[0] + halfWidth, y2: highPoint[1]
            },
            style: style
        }, {
            type: 'line',
            shape: {
                x1: highPoint[0], y1: highPoint[1],
                x2: lowPoint[0], y2: lowPoint[1]
            },
            style: style
        }, {
            type: 'line',
            shape: {
                x1: lowPoint[0] - halfWidth, y1: lowPoint[1],
                x2: lowPoint[0] + halfWidth, y2: lowPoint[1]
            },
            style: style
        }]
    };
  }
  