Mousemove for trending graph

svg.on("mousemove", function() {
    var x = d3.event.pageX - offsetLeft; 
    var beginning = x, end = pathLength, target;
    while (true) {
        target = Math.floor((beginning + end) / 2);
        pos = pathEl.getPointAtLength(target);
        pos.x += margin.left;
        pos.y += margin.top;
        if ((target === end || target === beginning) && pos.x !== x) {
            break;
        }
        if (pos.x > x)      end = target;
        else if (pos.x < x) beginning = target;
        else                break; //position found
    }
    if (pos.x >= xScale.range()[0] && pos.x <= xScale.range()[1] + margin.left) {
        var date = xScale.invert(pos.x);
        circle
          .attr("opacity", 1)
          .attr("cx", pos.x)
          .attr("cy", pos.y);
        tooltip.transition()        
          .duration(200)      
          .style("opacity", .9);      
        tooltip .html(formatTime(date) + "<br/>"  + getValueAt(date, data))  
                .style("left", d3.event.pageX + "px")     
                .style("top", (d3.event.pageY - 28) + "px");    
    }
});