how to add legend to a pie chart using D3js? And how to centralise the pie chart?

 <script type="text/javascript">
 var w = 400,                        //width
  h = 400,                            //height
  r = 150,                            //radius

  color = d3.scale.category20c();     //builtin range of colors


  data = [       
        {"label":"Single", "value":<?php echo $PercentageSingle; ?>}, 
        {"label":"In a relationship", "value":<?php echo $PercentageInRe; ?>}, 
        {"label":"Engaged", "value":<?php echo $PercentageEngaged; ?>},
        {"label":"Married", "value":<?php echo $PercentageMarried; ?>}, 
        {"label":"In an open relationship", "value":<?php echo $PercentageInOpenRe; ?>}, 
        {"label":"It's complicated", "value":<?php echo $PercentageCom; ?>}, 
        {"label":"Separated", "value":<?php echo $PercentageSeparated; ?>}, 
        {"label":"Divorced", "value":<?php echo $PercentageDivorced; ?>},
        {"label":"Widowed", "value":<?php echo $PercentageWidowed; ?>},
        {"label":"Unknown", "value":<?php echo $PercentageUnknown; ?>}        
];

var vis = d3.select("body")
    .append("svg:svg")              //create the SVG element inside the <body>
    .data([data])                   //associate our data with the document
        .attr("width", w)           //set the width and height of our visualization (these will be attributes of the <svg> tag
        .attr("height", h)
    .append("svg:g")                //make a group to hold our pie chart
        .attr("transform", "translate(" + r + "," + r + ")")    //move the center of the pie chart from 0, 0 to radius, radius

var arc = d3.svg.arc()              //this will create <path> elements for us using arc data
    .outerRadius(r);

var pie = d3.layout.pie()           //this will create arc data for us given a list of values
    .value(function(d) { return d.value; });    //we must tell it out to access the value of each element in our data array

var arcs = vis.selectAll("g.slice")     //this selects all <g> elements with class slice (there aren't any yet)
    .data(pie)                          //associate the generated pie data (an array of arcs, each having startAngle, endAngle and value properties) 
    .enter()                            //this will create <g> elements for every "extra" data element that should be associated with a selection. The result is creating a <g> for every object in the data array
        .append("svg:g")                //create a group to hold each slice (we will have a <path> and a <text> element associated with each slice)
            .attr("class", "slice");    //allow us to style things in the slices (like text)

    arcs.append("svg:path")
            .attr("fill", function(d, i) { return color(i); } ) //set the color for each slice to be chosen from the color function defined above
            .attr("d", arc);                                    //this creates the actual SVG path using the associated data (pie) with the arc drawing function

    arcs.append("svg:text")                                     //add a label to each slice
            .attr("transform", function(d) {                    //set the label's origin to the center of the arc
            //we have to make sure to set these before calling arc.centroid
            d.innerRadius = 0;
            d.outerRadius = r;
            return "translate(" + arc.centroid(d) + ")";        //this gives us a pair of coordinates like [50, 50]
        })
        .attr("text-anchor", "middle")                          //center the text on it's origin
        .text(function(d, i) { return data[i].label; });        //get the label from our original data array

</script>

I used the above code to generate a pie chart, however it's always at the left side of the web page when display, how can I centralise it? Also, when the slide is very small, the text will squeeze together, so how can i add legend to it instead of display the text inside each slide?