amcharts Line step

HTML

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

JavaScript

var chart;
var graph;

// months in JS are zero-based, 0 means January
var chartData = [{
    year: new Date(1950, 0,2),
    value: -0.307},
{
    year: new Date(1951, 0,3),
    value: -0.168},
{
    year: new Date(1952, 0,4),
    value: -0.073},
{
    year: new Date(1953, 0,5),
    value: -0.027},
{
    year: new Date(1954, 0,6),
    value: -0.251},
{
    year: new Date(1955, 0,7),
    value: -0.281},
{
    year: new Date(1956, 0,8),
    value: -0.348},
{
    year: new Date(1957, 0,9),
    value: -0.074},
{
    year: new Date(1958, 0,10),
    value: -0.011},
{
    year: new Date(1958, 0,13),
    value: -0.074},
{
    year: new Date(1958, 0,26),
    value: -0.124},
{
    year: new Date(1961, 0,23),
    value: -0.024},
{
    year: new Date(1961, 0,8),
    value: -0.022},
{
    year: new Date(1963, 0),
    value: 0.000},
{
    year: new Date(1964, 0),
    value: -0.296},
{
    year: new Date(1965, 0),
    value: -0.217},
{
    year: new Date(1966, 0),
    value: -0.147},
{
    year: new Date(1967, 0),
    value: -0.150},
{
    year: new Date(1968, 0),
    value: -0.160},
{
    year: new Date(1969, 0),
    value: -0.011},
{
    year: new Date(1970, 0),
    value: -0.068},
{
    year: new Date(1971, 0),
    value: -0.190},
{
    year: new Date(1972, 0),
    value: -0.056},
{
    year: new Date(1973, 0),
    value: 0.077},
{
    year: new Date(1974, 0),
    value: -0.213},
{
    year: new Date(1975, 0),
    value: -0.170},
{
    year: new Date(1976, 0),
    value: -0.254},
{
    year: new Date(1977, 0),
    value: 0.019},
{
    year: new Date(1978, 0),
    value: -0.063},
{
    year: new Date(1979, 0),
    value: 0.050},
{
    year: new Date(1980, 0),
    value: 0.077},
{
    year: new Date(1981, 0),
    value: 0.120},
{
    year: new Date(1982, 0),
    value: 0.011},
{
    year: new Date(1983, 0),
    value: 0.177},
{
    year: new Date(1984, 0),
    value: -0.021},
{
    year: new Date(1985, 0),
    value: -0.037},
{
    year: new Date(1986, 0),
    value:...