If you want to add legend in your graph as shown in below image, just try to use below code :
// LEGEND
var legend = new AmCharts.AmLegend();
legend.position = "bottom"; // you can set 'left', 'right','top' as well
legend.markerType = "bubble";
legend.markerSize = 7;
//legend.borderAlpha = 0.3;
legend.horizontalGap = 10;
//legend.switchType = "v";
legend.align ="center";
chart.addLegend(legend);
// LEGEND
var legend = new AmCharts.AmLegend();
legend.position = "bottom"; // you can set 'left', 'right','top' as well
legend.markerType = "bubble";
legend.markerSize = 7;
//legend.borderAlpha = 0.3;
legend.horizontalGap = 10;
//legend.switchType = "v";
legend.align ="center";
chart.addLegend(legend);