V3: Line Step no risers

HTML

<script src="http://www.amcharts.com/lib/3/amcharts.js"></script>
<script src="http://www.amcharts.com/lib/3/serial.js"></script>
<div id="chartdiv" style="width: 100%; height: 362px;"></div>

JavaScript

var chart;
var graph;

var chartData = [
    {
        "year": "1950",
        "value": 0.22
    },
    {
        "year": "1951",
        "value": 0.168
    },
    {
        "year": "1952",
        "value": 0.103
    },
    {
        "year": "1953",
        "value": 0.067
    },
    {
        "year": "1954",
        "value": 0.151
    },
    {
        "year": "1955",
        "value": 0.281
    },
    {
        "year": "1956",
        "value": 0.348
    },
    {
        "year": "1957",
        "value": 0.274
    },
    {
        "year": "1958",
        "value": 0.211
    },
    {
        "year": "1959",
        "value": 0.174
    },
    {
        "year": "1960",
        "value": 0.124
    },
    {
        "year": "1961",
        "value": 0.064
    },
    {
        "year": "1962",
        "value": 0.032
    },
    {
        "year": "1963",
        "value": 0.05
    },
    {
        "year": "1964",
        "value": 0.106
    }
];


AmCharts.ready(function () {
    // SERIAL CHART
    chart = new AmCharts.AmSerialChart();
    chart.pathToImages = "http://www.amcharts.com/lib/3/images/";
    chart.marginRight = 0;
    chart.marginTop = 0;
    chart.dataProvider = chartData;
    chart.categoryField = "year";
    chart.dataDateFormat = "YYYY";
    chart.color = "#0000";
    
    chart.backgroundImage = "http://www.amcharts.com/lib/3/images/bgSky.jpg";
    
    // AXES
    // Category
    var categoryAxis = chart.categoryAxis;
    categoryAxis.parseDates = true; // as our data is date-based, we set parseDates to true
    categoryAxis.minPeriod = "YYYY"; // our data is yearly, so we set minPeriod to YYYY
    categoryAxis.gridCount = 20;
    categoryAxis.autoGridCount = false;
    categoryAxis.axisAlpha = 0.3;
    categoryAxis.axisColor = "#FFFFFF";
    categoryAxis.gridAlpha = 0.05;
    
    // VALUE
    var valueAxis = new AmCharts.ValueAxis();
    valueAxis.gridAlpha = 0;
    valueAxis.axisAlpha = 0.3;
    valueAxis.axisColor = "#FFFFFF";
    valueAxis.showLastLabel =...