Zoom + Pan

HTML

<script src="http://d3js.org/d3.v3.min.js"></script>
<script src="http://underscorejs.org/underscore-min.js"></script>
<div id="area"></div>

CSS

.plot {
    fill: rgba(250, 250, 255, 0.6);
}
.grid .tick {
    stroke: lightgrey;
    opacity: 0.7;
}
.grid path {
    stroke-width: 0;
}
.axis path, .axis line {
    fill: none;
    stroke: #000;
    shape-rendering: crispEdges;
}
.x.axis path {
    display: none;
}
.line {
    fill: none;
    stroke: steelblue;
    stroke-width: 1.5px;
}

JavaScript

var data = [{"item":1,"diameter":"15.00","point":"8.00"},{"item":2,"diameter":"10.00","point":"2.00"},
{"item":3,"diameter":"25.00","point":"2.00"},{"item":4,"diameter":"7.00","point":"1.00"},
{"item":5,"diameter":"35.00","point":"8.00"},{"item":6,"diameter":"15.00","point":"2.00"},
{"item":7,"diameter":"12.00","point":"2.00"},{"item":8,"diameter":"10.00","point":"4.00"},
{"item":9,"diameter":"17.00","point":"1.00"},{"item":10,"diameter":"13.00","point":"3.00"},
{"item":11,"diameter":"5.00","point":"5.00"},{"item":12,"diameter":"10.00","point":"6.00"},
];
            
function line_chart(field,el){ 
  margin = {
    top: 20,
    right: 20,
    bottom: 20,
    left: 45
  };
  tickno = 8;
  
  width = 960 - margin.left - margin.right;
  height = 400 - margin.top - margin.bottom;

var x = d3.scale.linear().domain(d3.extent(data, function (d) {
    return d.item;
}))
    .range([0, width]);
var ymax = d3.max(data,function(d){
    return (parseInt(d[field])+1);
});
var ymin = d3.min(data,function(d){
    return d[field];
});

var xmax = d3.max(data,function(d){
    return d.name;
});
var y = d3.scale.linear()
    .domain([ymin,ymax])
    .range([height, 0]);

var line = d3.svg.line()
    .x(function (d) {
    return x(d.item);
})
    .y(function (d) {
    return y(d[field]);
});

var zoom = d3.behavior.zoom()
    .x(x)
    .scaleExtent([-1, 2])
    .y(y)
    .on("zoom", zoomed);

svg = d3.select(el)
    .append("svg:svg")
    .attr('width', width + margin.left + margin.right)
    .attr('height', height + margin.top + margin.bottom)
    .append("svg:g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")")
    .call(zoom);

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

var make_x_axis = function () {
    return d3.svg.axis()
        .scale(x)
        .orient("bottom")
        .ticks(tickno);
};

var make_y_axis = function () {
    return d3.svg.axis()
        .scale(y)
       ...