Draw a D3 circle with gradient colours

How to draw a circle with gradient color? Say, a gradient from yellow to blue.

Normally, to create a circle in yellow we can use the following code:

var cdata=[50,40];
var xscale=40;
var xspace =50;
var yscale=70;

var svg = d3.select("body")
    .append("svg")
    .attr("width", 1600)
    .attr("height", 1600);

var circle = svg.selectAll("circle")
    .data(cdata)
   .enter()
    .append("circle");

var circleattr = circle
    .attr("cx", function(d) {
        xscale = xscale+xspace;
        return xscale;
    })
    .attr("cy", function(d) {
        yscale=yscale+xspace+10;
        return yscale;
    })
    .attr("r", function(d) {
        return d;
    })
    .style("fill","yellow");