D3 Examples - The dots shows only after hovering the continent dots

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<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width">
    <script src="https://d3js.org/d3.v4.min.js"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/d3-legend/2.24.0/d3-legend.min.js"></script>
    <title>Basic Scatter Plot</title>
    <style>
        body {
            margin: 0px;
        }
​
        .domain {
            display: none;
        }
​
        .tick line {
            stroke: #C0C0BB;
        }
​
        .tick text, .legendCells text {
            fill: #8E8883;
            font-size: 13pt;
            font-family: sans-serif;
        }
​
        .axis-label, .legend-label {
            fill: #635F5D;
            font-size: 15pt;
            font-family: sans-serif;
        }
    </style>
</head>
<body>
<svg width="960" height="500"></svg>
<script>
​
    const xValue = d => d.income;
    const xLabel = 'Income Per Person, ($/year)';
    const yValue = d => d.health;
    const yLabel = 'Health Expectancy, (years)';
    const rValue = d => d.population;
    const rLabel = 'Size of spots stand for population';
    const colorLabel = 'region';
    const margin = {left: 120, right: 250, top: 30, bottom: 100};
​
    const svg = d3.select('svg');
    const width = svg.attr('width');
    const height = svg.attr('height');
    const innerWidth = width - margin.left - margin.right;
    const innerHeight = height - margin.top - margin.bottom;
​
    let hoveredColorValue;
    const hoveredStrokeColor = "black";
​
    const g = svg.append('g')
        .attr('transform', `translate(${margin.left},${margin.top})`);
​
    // This is the layer where the bars are drawn.
    const baseBarLayer = g.append("g");
​
​
    // This layer contains a semi-transparent overlay
    // that fades out the base bars.
    const overlayRect = g.append("g")
        .append("rect")
        .attr("width", innerWidth)
        .attr("height", innerHeight)
        .attr("fill", "none")
        .style("pointer-events", "none");
​
    // This contains the subset of bars rendered on top
    // when you hover over the entries in the color legend.
​
​
    const foregroundBarLayer = g.append("g");
​
​
    const xAxisG = g.append('g')
        .attr('transform', `translate(0, ${innerHeight})`);
    const yAxisG = g.append('g');
    const colorLegendG = g.append('g')
        .attr("class", "color-legend")
        .attr('transform', `translate(${innerWidth + 60}, 150)`);
​
    xAxisG.append('text')
        .attr('class', 'axis-label')
        .attr('x', innerWidth / 2)
        .attr('y', 70)
        .text(xLabel);
​
    yAxisG.append('text')
        .attr('class', 'axis-label')
        .attr('x', -innerHeight / 2)
        .attr('y', -60)
        .attr('transform', `rotate(-90)`)
        .style('text-anchor', 'middle')
        .text(yLabel);
​
    colorLegendG.append('text')
        .attr('class', 'legend-label')
        .attr('x', -30)
        .attr('y', -40);
​
    const xScale = d3.scaleLog();
    const yScale = d3.scaleLinear();
    const rScale = d3.scaleSqrt();
    const colorScale = d3.scaleOrdinal()
        .range(d3.schemeCategory10);
​
    const xAxis = d3.axisBottom()
        .scale(xScale)
        .ticks(11, ",.1s")
        .tickPadding(15)
        .tickSize(-innerHeight);
​
    const yAxis = d3.axisLeft()
        .scale(yScale)
        .ticks(5)
        .tickPadding(15)
        .tickSize(-innerWidth);
​
    const colorLegend = d3.legendColor()
        .scale(colorScale)
        .shape('circle');
​
    const row = d => {
        d.income = +d.income;
        d.health = +d.health;
        d.population = +d.population;
        return d;
    };
​
    function render(data) {
​
        const nested = d3.nest()
            .key(function (d) {
                return d[colorLabel];
            })
            .entries(data);
​
        xScale
            .domain(d3.extent(data, xValue))
            .range([0, innerWidth])
            .nice;
​
        yScale
            .domain(d3.extent(data, yValue))
            .range([innerHeight, 0])
            .nice();
​
        rScale
            .domain(d3.extent(data, rValue))
            .range([4, 18]);
​
        colorScale.domain(nested.map(function (nested) {
            return nested.key;
        }));
​
        xAxisG.call(xAxis);
        yAxisG.call(yAxis);
​
        renderBars(baseBarLayer, nested);
​
​
        if (hoveredColorValue) {
            setOverlayTransparency(0.8);
            renderBars(foregroundBarLayer, nested.filter(function (layer) {
                return layer.key === hoveredColorValue;
            }));
        } else {
            setOverlayTransparency(0.0);
            renderBars(foregroundBarLayer, nested);
        }
​
        colorLegendG.call(colorLegend)
            .selectAll('.cell text')
            .attr('dy', '0.1em');
​
        listenForHover(colorLegendG.selectAll("circle"), data);
        listenForHover(colorLegendG.selectAll("text"), data);
    }
​
    function renderBars(g, layers) {
        const layerGs = g.selectAll(".layer").data(layers);
        layerGs.enter().append("g").attr("class", "layer");
        layerGs.exit().remove();
​
        layerGs.style("fill", function (d) {
            return colorScale(d.key);
        });
​
        const circles = layerGs.selectAll('.mark').data(function (d) {
            return d.values;
        });
​
        circles.enter().append('circle')
            .attr('class', 'mark')
            .merge(circles)
            .attr('cx', d => xScale(xValue(d)))
            .attr('cy', d => yScale(yValue(d)))
            .attr('fill-opacity', 0.8)
            .attr('r', d => rScale(rValue(d)));
​
        circles.exit().remove();
​
    }
​
    function listenForHover(selection, data) {
        selection
            .on("mouseover", function (d) {
                hoveredColorValue = d;
                render(data);
            })
            .on("mouseout", function (d) {
                hoveredColorValue = null;
                render(data);
            })
            .style("cursor", "pointer");
    }
​
    function setOverlayTransparency(alpha) {
        overlayRect
            .transition().duration(400)
            .attr("fill", "rgba(255, 255, 255, " + alpha + ")");
    }
​
    d3.csv('income_health.csv', row, render);
</script>
</body>
</html>
    