var editor = CodeMirror.fromTextArea(document.getElementById("code"), {

  lineNumbers: true,

  gutters: ["CodeMirror-linenumbers", "breakpoints"]

});

editor.on("gutterClick", function(cm, n) {

  var info = cm.lineInfo(n);

  cm.setGutterMarker(n, "breakpoints", info.gutterMarkers ? null : makeMarker());

});

​

function makeMarker() {

  var marker = document.createElement("div");

  marker.style.color = "#822";

  marker.innerHTML = "●";

  return marker;

}

​