Floating column chart spanning from negative to positive with threshold marker

HTML

<script src="http://www.amcharts.com/lib/amcharts.js" type="text/javascript"></script>
<div id="chartdiv" style="width: 100%; height: 355px; background-color: white;"></div>

JavaScript

var chart;

var chartData = [{
    country: "1 month",
    q1from: -2,
    q1to: 3,
    q2from: 3,
    q2to: 5,
    q3from: 5,
    q3to: 6,
    q4from: 6,
    q4to: 7,
    div: 3
}, {
    country: "3 months",
    q1from: -1,
    q1to: 2,
    q2from: 2,
    q2to: 6,
    q3from: 6,
    q3to: 6.5,
    q4from: 6.5,
    q4to: 7.3,
    div: 6
}, {
    country: "6 month",
    q1from: 4,
    q1to: 4.2,
    q2from: 4.2,
    q2to: 4.4,
    q3from: 4.4,
    q3to: 5,
    q4from: 5,
    q4to: 7.5,
    div: 4.4
}, {
    country: "3 years",
    q1from: 3.5,
    q1to: 3.7,
    q2from: 3.7,
    q2to: 4,
    q3from: 4,
    q3to: 5,
    q4from: 5,
    q4to: 6.8,
    div: 3.7
}, ];


AmCharts.ready(function () {
    // SERIAL CHART
    chart = new AmCharts.AmSerialChart();
    chart.dataProvider = chartData;
    chart.categoryField = "country";
    chart.startDuration = 1;

    // AXES
    // category
    var categoryAxis = chart.categoryAxis;
    //categoryAxis.axisAlpha = 0;
    //categoryAxis.labelRotation = 90;
    //categoryAxis.gridPosition = "start";

    // value
    var valueAxis = new AmCharts.ValueAxis();
    valueAxis.unit = '%';
    valueAxis.unitPosition = 'right';
    valueAxis.stackType = "3d";
    chart.addValueAxis(valueAxis);

    // GRAPH

    var graph_div = new AmCharts.AmGraph();
    graph_div.openField = "div";
    graph_div.valueField = "div";
    graph_div.type = "column";
    graph_div.lineAlpha = 1;
    graph_div.lineThickness = 2;
    graph_div.fillAlphas = 1;
    graph_div.fillColors = "#FF6419";
    chart.addGraph(graph_div);

    var graph_q4 = new AmCharts.AmGraph();
    graph_q4.openField = "q4from";
    graph_q4.valueField = "q4to";
    graph_q4.type = "column";
    graph_q4.lineAlpha = 0;
    graph_q4.fillAlphas = 1;
    graph_q4.fillColors = "#275737";
    chart.addGraph(graph_q4);

    var graph_q3 = new AmCharts.AmGraph();
    graph_q3.openField = "q3from";
    graph_q3.valueField = "q3to";
    graph_q3.type = "column";
    graph_q3.lineAlpha =...