A key concept in D3 is the idea of binding data to nodes, which you can do with the datum() function.

This associates a data object with the node, and that data object is passed as the first parameter to every attribute function that is called on that object.

For example:

var data = [{"x": 200}];
var circle = g.append("circle").datum(data);

binds the data variable to the circle, and

circle.attr("cx", function(d){ return d.x;});

passes the datum associated with the circle as the parameter d into that function.