Write the function updateEvenCircles that will take a selection of
elements, and update the data bound
to circles with even keys to the values in the variable newData.
Each circle is bound to an object with the shape {key: ..., value: ...}, and the join selection
corresponding to the key field of each value in the array.
<body>
<svg id="svg">
</svg>
<script>
var elements = d3.selectAll("#svg")
.data([{ key: "4", value: 6},
{ key: "3", value: 12},
{ key: "2", value: 5},
{ key: "1", value: 10}],
function(d) { return d.key; })
.enter().append("circle");
data = [{ key: "2", value: 100},
{ key: "4", value: 200}];
updateEvenCircles(elements, data); // write this function
</script>
</body>