Adding tooltip to line chart not showing

I made a line chart, added bullets on nodal points. Now, I just want to add some tooltips to be shown on hovering the points.

The text seems to be added, but I cannot see any value (must be of frequency) on hovering nodal points.

SNIPPET:

<html>

<head>

    <script src="https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.4.12/angular.min.js"></script> 
    <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.1.1/jquery.min.js"></script> 
    <script src="https://cdnjs.cloudflare.com/ajax/libs/d3/4.3.0/d3.min.js"></script>

</head> 

<body ng-app="myApp" ng-controller="myCtrl"> 

    <svg></svg>

    <script>

        //module declaration 
        var app = angular.module('myApp',[]);

        //Controller declaration
        app.controller('myCtrl',function($scope){

            $scope.svgWidth = 800;//svg Width
            $scope.svgHeight = 500;//svg Height 

            //Data in proper format 
            var data = [
                  {"letter": "A","frequency": "5.01"},
                  {"letter": "B","frequency": "7.80"},
                  {"letter": "C","frequency": "15.35"},
                  {"letter": "D","frequency": "22.70"},
                  {"letter": "E","frequency": "34.25"},
                  {"letter": "F","frequency": "10.21"},
                  {"letter": "G","frequency": "7.68"},
            ];

                //removing prior svg elements ie clean up svg 
                d3.select('svg').selectAll("*").remove();

                //resetting svg height and width in current svg 
                d3.select("svg").attr("width", $scope.svgWidth).attr("height", $scope.svgHeight);

                //Setting up of our svg with proper calculations 
                var svg = d3.select("svg");
                var margin = {top: 20, right: 20, bottom: 30, left: 40};
                var width = svg.attr("width") - margin.left - margin.right;
                var height = svg.attr("height") - margin.top - margin.bottom;

                //Plotting our base area in svg in which chart will be shown 
                var g = svg.append("g").attr("transform", "translate(" + margin.left + "," + margin.top + ")");

                //X and Y scaling 
                var x = d3.scaleBand().rangeRound([0, width]).padding(0.4);
                var y = d3.scaleLinear().rangeRound([height, 0]);

                x.domain(data.map(function(d) { return d.letter; }));
                y.domain([0, d3.max(data, function(d) { return +d.frequency; })]);

                //Final Plotting 

                //for x axis 
                g.append("g")
                    .call(d3.axisBottom(x))
                    .attr("transform", "translate(0," + height + ")");

                //for y axis 
                g.append("g")
                    .call(d3.axisLeft(y))
                    .append("text").attr("transform", "rotate(-90)").attr("text-anchor", "end");

                //the line function for path 
                var lineFunction = d3.line()
                    .x(function(d) {return x(d.letter); })
                    .y(function(d) { return y(d.frequency); })
                    .curve(d3.curveCardinal);

                //defining the lines
                var path = g.append("path");

                //plotting lines
                path
                    .attr("d", lineFunction(data))
                    .attr("stroke", "#fc9027")
                    .attr("stroke-width", 2)
                    .attr("fill", "none");

                g.selectAll('.circles1')
                    .data(data)
                    .enter().append('circle')
                    .attr('cx', function(d) {
                    return x(d.letter);
                    })
                    .attr('cy', function(d) {
                    return y(d.frequency);
                    })
                    .attr('r', 6)
                    .style("fill", "#fc9027")
                    .on("mouseover", function(d,i) {
                        d3.select(this).append("text")
                        .text(d.frequency)
                        .attr("x", x(d.letter))
                        .attr("y", y(d.frequency))
                    }); 

        });

    </script> 

</body> 

</html>

RESULT:

Solution

When you do this:

d3.select(this).append("text")

You're appending a <text> element to a <circle> element, and this will simply not work.

There are alternatives. The simplest one is appending a <title>:

 d3.select(this).append("title")

But a title is very limited. Alternatively, you can use a div to show the tooltip (check my answer here: https://stackoverflow.com/a/35665974/5768908)