Gradient colors from d3.scale.category10() with opacity change on a svg circle? - svg

Question

I am trying to apply the colors from the color = d3.scale.category10(); var to the gradient for the circle svg, what am I doing wrong? All I am seeing is the first color of the color = d3.scale.category10();(which is blue) to 0% opacity gradient but that is all. If I take the gradient out then I see the range I want which is from 1-4? Thanks in advance!
var nodes = d3.range(300).map(function() { return {radius: Math.random() * 12 + 4}; }),
root = nodes[0],
color = d3.scale.category10();
root.radius = 0;
root.fixed = true;
var force = d3.layout.force()
.gravity(0.05)
.charge(function(d, i) { return i ? 0 : -4000; })
.nodes(nodes)
.size([width, height]);
force.start();
var svg = d3.select("body").append("svg:svg")
.attr("width", width)
.attr("height", height);
var gradient = svg.append("defs").append("radialGradient")
.attr("id", "gradient")
.attr("cx", "50%")
.attr("cy", "50%");
gradient.append("stop")
.attr("offset", "75%")
.style("stop-color", function(d, i) { return color(i % 4); })
.attr("stop-opacity", "1");
gradient.append("stop")
.attr("offset", "100%")
.style("stop-color", function(d, i) { return color(i % 4); })
.attr("stop-opacity", ".1");
svg.selectAll("circle")
.data(nodes.slice(1))
.enter().append("circle")
.attr("r", function(d) { return d.radius; })
.style("fill", "url(#gradient)");

Answer 1

Your stop elements don't have any data joined with them, so in your function (d, i), i will always be 0. If you just want the two stops, you could do something like this:
gradient.append("stop")
.attr("offset", "75%")
.style("stop-color", color(0))
.attr("stop-opacity", "1");
gradient.append("stop")
.attr("offset", "100%")
.style("stop-color", color(1))
.attr("stop-opacity", ".1");
If instead you're just trying to fade the edges of your circles, a gradient isn't what you want at all. Instead, you'll need to apply a solid color to each circle, then create a single opacity-only gradient inside a mask, and apply that mask to each circle. Something like this:
var defs = svg.append('defs');
var gradient = defs.append('radialGradient')
.attr('id', 'fadient');
gradient.append('stop')
.attr('offset', '75%')
.attr('stop-color', 'white')
.attr('stop-opacity', 1)
gradient.append('stop')
.attr('offset', '100%')
.attr('stop-color', 'white')
.attr('stop-opacity', .1)
var mask = defs.append('mask')
.attr('id', 'mask')
.attr('maskContentUnits', 'objectBoundingBox')
.append('circle')
.attr('fill', 'url(#fadient)')
.attr('cx', .5)
.attr('cy', .5)
.attr('r', .5)
svg.selectAll("circle")
.data(nodes.slice(1))
.enter().append("circle")
.attr('cx', function (d, i) { return 20 * i })
.attr('cy', 50)
.attr("r", function(d) { return d.radius; })
.attr('mask', 'url(#mask)')
.attr("fill", function (d, i) { return color(i); });