Zoom & Pan (only X)

HTML

<script src="http://d3js.org/d3.v3.min.js"></script>

CSS

body {
    font: 12px Arial;
    background-color: #000000;
}

path {
    stroke: yellow;
    stroke-width: 1;
    fill: none;
}
.axis path, .axis line {
    fill: none;
    stroke: #ccc;
    stroke-width: 2;
    shape-rendering: crispEdges;
}

 text {
    fill: #ccc;
}

rect {
    fill:  #000000;
}

JavaScript

var lastUpdateTime = + new Date();

var GenData = function(N, lastTime){
    var output = [];
    	var level=1000;
    for (var i=0; i<N; i++){

        output.push({value:level, timestamp:lastTime});
        lastTime = lastTime + 1000;    
    }
    return output;
}

var globalData;
var dataIntervals = 1;

// plot the original data by retrieving everything from time 0
data = GenData(100, lastUpdateTime);
lastUpdateTime = data[data.length-1].timestamp;

globalData = data;

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

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

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


x.domain(d3.extent(globalData, function (d) { return d.timestamp; }));
y.domain(d3.extent(globalData, function (d) { return d.value;}));


var xAxis = d3.svg.axis().scale(x)
    .orient("bottom")
    .ticks(d3.time.seconds, 20)
    .tickFormat(d3.time.format('%X'))
    .tickSize(1)
    .tickPadding(8);

var xAxisTop = d3.svg.axis().scale(x)
    .orient("bottom").tickFormat("").tickSize(0);

var yAxis = d3.svg.axis().scale(y)
    .orient("left").ticks(5);

var yAxisRight = d3.svg.axis().scale(y)
    .orient("right").tickFormat("").tickSize(0);

var valueline = d3.svg.line()
    .x(function (d) { return x(d.timestamp); })
    .y(function (d) { return y(d.value); });

var zoom = d3.behavior.zoom()
    .x(x)
    .y(y)
    .scaleExtent([1, 4])
    .on("zoom", zoomed);

var svg = d3.select("body")
    .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 + ")")
    .call(zoom);

svg.append("rect")
    .attr("width", width)
    .attr("height", height)
    .attr("class", "plot"); // ????

var clip = svg.append("clipPath")
    .attr("id", "clip")
    .append("rect")
    .attr("x", 0)
    .attr("y",...