var fromX, fromY;
var svg = document.createElementNS ('http://www.w3.org/2000/svg',"svg");
svg.setAttribute("style", "position: absolute; top:0;left:0;height: " + document.body.clientHeight + "px;width: 100%");
var line = document.createElementNS('http://www.w3.org/2000/svg','line');
line.setAttribute("style", "stroke-width: 4; stroke: red");
svg.appendChild(line);
document.body.appendChild(svg);
document.body.addEventListener("mousedown", function (e) { fromX = e.pageX; fromY = e.pageY;
});
document.body.addEventListener("mousemove", function (e) { if (fromX === undefined) { return; } line.setAttribute("x1", fromX); line.setAttribute("x2", e.pageX); line.setAttribute("y1", fromY); line.setAttribute("y2", e.pageY); console.log( [fromX, fromY], " to ", [e.pageX, e.pageY], "Distance:", Math.sqrt(Math.pow(fromX - e.pageX, 2) + Math.pow(fromY - e.pageY, 2)) );
});
document.body.addEventListener("mouseup", function (e) { fromX = undefined; fromY = undefined;
});