How to animate line graph in d3.js?

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var margin = {top: 20, right: 20, bottom: 30, left: 50},
    width = 600 - margin.left - margin.right,
    height = 300 - margin.top - margin.bottom;

var svg = d3.select("#chart")
  .append("svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
  .append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

var xScale = d3.scaleLinear()
    .range([0, width]);

var yScale = d3.scaleLinear()
    .range([height, 0]);

var xAxis = d3.axisBottom(xScale);

var yAxis = d3.axisLeft(yScale);

svg.append("g")
    .attr("class", "x axis")
    .attr("transform", "translate(0," + height + ")");

svg.append("g")
    .attr("class", "y axis");

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var line = d3.line()
    .x(function(d) { return xScale(d.x); })
    .y(function(d) { return yScale(d.y); });

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function update(data) {
  xScale.domain(d3.extent(data, function(d) { return d.x; }));
  yScale.domain([0, d3.max(data, function(d) { return d.y; })]);

  svg.select(".x.axis")
    .transition()
    .call(xAxis);

  svg.select(".y.axis")
    .transition()
    .call(yAxis);

  var path = svg.selectAll("path.line")
    .data([data]);

  path.enter().append("path")
    .attr("class", "line")
    .merge(path)
    .transition()
    .attr("d", line);
}

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var data = [
  { x: 0, y: 10 },
  { x: 1, y: 20 },
  { x: 2, y: 15 },
  { x: 3, y: 25 },
  { x: 4, y: 30 }
];

update(data);