Line transition

Using the MCFC dataset the above visual shows how the ball was moved around the pitch during 90 minutes of play. The curve is not contained in the dataset but added for effect. One thing you'll learn from above is that reducing opacity of the svg path (opacity:0.9; in style section at the top) does not have the desired effect. Overlapping lines would not get darker. For this to work the paths would have to be separate elements.

Links to data: http://www.optasports.com/news-area/optapro-and-manchester-city-launch-mcfc-analytics.aspx

<!DOCTYPE HTML>
<html>
<head>
<meta charset="utf-8">
<title>line growing example d3js</title>
<script src="https://d3js.org/d3.v3.min.js"></script>

<style>

.line {
    fill: none;
    stroke-width: 1;
    stroke:#0099FF;
    opacity:0.9;
}

</style>
</head>
<body>
<script type="text/javascript">

var svg = d3.select("body")
            .append("svg")
            .attr("width", 1500)   
            .attr("height", 1500); 
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var x = d3.scale.linear().domain([0,100]).range([40,400]);
var y = d3.scale.linear().domain([0,100]).range([60,600]);

}

)

</script>

</body>
</html>

Modified http://d3js.org/d3.v3.min.js to a secure url

https://d3js.org/d3.v3.min.js