Axis transition

<!DOCTYPE html>
<html>

<head>
    <meta charset="utf-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
    <title>Examples</title>
    <meta name="description" content="">
    <meta name="keywords" content="">
    <link href="" rel="stylesheet">
    <style type="text/css">
    path.line {
        fill: none;
        stroke-width: 2px;
    }
    path.domain {
        fill: none;
        stroke: black;
        stroke-width: 1px;
        shape-rendering: crispEdges;
    }
    .choosen {
        fill: none;
        stroke: green;
        stroke-width: 2px;
        shape-rendering: crispEdges;
    }
    text {
        font-family: arial;
    }
    </style>

</head>

</body>
<script type="text/javascript">
var margin = {
    top: 20,
    right: 30,
    bottom: 30,
    left: 60
},
    width = 960 - margin.left - margin.right,
    height = 350 - margin.top - margin.bottom;

var stateData;

var x = d3.scale.linear()
    .range([0, width]);

var y = d3.scale.linear()
    .range([height, 0]);

var xAxis = d3.svg.axis()
    .scale(x)
    .tickFormat(d3.format(".0f"))
    .orient("bottom");

var yAxis = d3.svg.axis()
    .scale(y)
    .tickFormat(d3.format(",.0f"))
    .orient("left");

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 line = d3.svg.line()
    .interpolate("linear")
    .x(function(d) {
        return x(+d.date);
    })
    .y(function(d) {
        return y(+d.thing);
    });

d3.tsv('data.tsv', function(error, dataset) {

var names = d3.keys(dataset[0]).filter(function(key) {
        return key !== "Date";
    });

stateData = states;

d3.select("#dropDown")
        .append("select")
        .on("change", function() {
            change(this.options[this.selectedIndex].__data__);
        })
        .selectAll("option")
        .data(names.filter(function(d) {
            return d !== "United States";
        })).enter()
        .append("option")
        .text(function(d) {
            return d;
        });

x.domain(d3.extent(dataset, function(d, i) {
        return +d.Date;
    }));

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

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

var stateLines = svg.selectAll(".states")
        .data(states).enter()
        .append("path")
        .attr("class", "line")
        .attr("d", function(d) {
            return line(d.values);
        })
        .attr("stroke", function(d, i) {
            return (d.name === "United States") ? "orange" : "grey";
        });

var usFilter = stateFilter("United States");

var maxUS = dataMax(usFilter);

change("Alabama");

function change(x) {

var data = stateFilter(x);

var maxNew = Math.max(dataMax(data), maxUS);

var greenLine = svg.selectAll(".choosen")
            .data(data);

y.domain([0, maxNew]);

greenLine.transition()
            .duration(2000)
            .attr("d", function(d) {
                return line(d.values);
            });

greenLine.enter().append("path")
            .attr("class", "choosen")
            .attr("d", function(d) {
                return line(d.values);
            });

stateLines.transition().duration(2000)
            .attr("d", function(d) {
                return line(d.values);
            });

greenLine.exit()
            .transition()
            .duration(2000)
            .remove();

svg.selectAll(".y")
            .transition().duration(2000)
            .call(yAxis.scale(y));
    }

function stateFilter(x) {
        var ans = states.filter(function(d) {
            return d.name === x;
        });
        return ans
    }
})
</script>

</html>

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

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