Added pyramid chart layout built in d3.js

pyramid/README.md

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+# Pyramid Chart Layout built in d3.js
+
+Demo: <http://bl.ocks.org/ronakrrb/73e9204a66e2a9c1fee8>
+
+```js
+var pyramid = d3.pyramid()
+    .size([width,height])
+    .value(function(d) { return d.value; });
+```
+
+```
+var g = d3.selectAll(".pyramid-group")
+    .data(pyramid(data))
+    .enter().append("g")
+    .attr("class", "pyramid-group");
+```

pyramid/pyramid.js

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+d3.pyramid = function() {
+  var size = [1,1],
+      value = function(d) { return d.value},
+      coordinates;
+
+  var percentageValues = function (data) {
+    var values = data.map(value);
+    var sum = d3.sum(values, function (d){
+      return +d;
+    });
+    var percentValues = data.map(function (d,i){
+      d.value = +values[i];
+      return values[i]/sum*100;
+    });
+    percentValues.sort(function(a,b){
+      return b-a;
+    });
+    return percentValues;
+  }
+  var coordinatesCalculation = function(data){
+    var w = size[0],
+        h = size[1],
+        ratio = (w/2)/h,
+        percentValues = percentageValues(data),
+        coordinates = [],
+        area_of_triangle = (w * h) / 2;
+    function d3Sum (i) {
+      return d3.sum(percentValues,function (d, j){
+        if (j>=i) {
+          return d;
+        }
+      });
+    }
+    for (var i=0,len=data.length;i<len; i++){
+      var selectedPercentValues = d3Sum(i),
+          area_of_element = selectedPercentValues/100 * area_of_triangle,
+          height1 = Math.sqrt(area_of_element/ratio),
+          base = 2 * ratio * height1,
+          xwidth = (w-base)/2;
+      if (i===0){
+        coordinates[i] = {"values":[{"x":w/2,"y":0},{"x":xwidth,"y":height1},{"x":base+xwidth,"y":height1}]};
+      }else{
+        coordinates[i] = {"values":[coordinates[i-1].values[1],{"x":xwidth,"y":height1},{"x":base+xwidth,"y":height1},coordinates[i-1].values[2]]};
+      }
+
+    }
+    coordinates[0].values[1] = coordinates[coordinates.length-1].values[1];
+    coordinates[0].values[2] = coordinates[coordinates.length-1].values[2];
+    var first_data = coordinates.splice(0,1);
+    coordinates = coordinates.reverse();
+    coordinates.splice(0,0,first_data[0]);
+    return coordinates;
+  } 
+  function pyramid(data) {
+    var i = 0,
+        coordinates = coordinatesCalculation(data);
+
+    data.sort(function(a,b) {
+      return a.value - b.value;
+    })
+
+    data.forEach(function(){
+      data[i].coordinates = coordinates[i].values;
+      i++;
+    })
+    return data;
+  }
+  pyramid.size = function(s){
+    if(s.length === 2) {
+      size = s;                    
+    }
+    return pyramid;
+  }
+  pyramid.value = function(v) {
+    if (!arguments.length) return value;
+    value = v;
+    return pyramid;
+  };
+  return pyramid;
+}