svg multiple color on circle stroke

I want to create a rainbow circle, like the picture below:

How can I draw the curved and multiple color stop gradient?

Here's my current code:

<svg width="500" height="500" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
    <defs>
    <linearGradient id="test">
    <stop offset="0%" stop-color="#f00"/>
    <stop offset="100%" stop-color="#0ff"/>
    </linearGradient>    
    </defs>    
    <circle cx="50" cy="50" r="40" fill="none" stroke="url(#test)" stroke-width="6"/>    
</svg>

Solution

This approach won't work. SVG doesn't have conical gradients. To simulate the effect, you would have to fake it with a large number of small line segments. Or some similar technique.

Update:

Here is an example. I approximate the 360deg of hue with six paths. Each path contains an arc which covers 60deg of the circle. I use a linear gradient to interpolate the colour from the start to the end of each path. It's not perfect (you can see some discontinuities where the coloursmeet ) but it would possibly fool most people. You could increase the accuracy by using more than six segments.

    <svg xmlns="http://www.w3.org/2000/svg" version="1.1" width="100%" height="100%" viewBox="-10 -10 220 220">
      <defs>
        <linearGradient id="redyel" gradientUnits="objectBoundingBox" x1="0" y1="0" x2="1" y2="1">
            <stop offset="0%" stop-color="#ff0000"/>   
            <stop offset="100%" stop-color="#ffff00"/>   
        </linearGradient>
        <linearGradient id="yelgre" gradientUnits="objectBoundingBox" x1="0" y1="0" x2="0" y2="1">
            <stop offset="0%" stop-color="#ffff00"/>   
            <stop offset="100%" stop-color="#00ff00"/>   
        </linearGradient>
        <linearGradient id="grecya" gradientUnits="objectBoundingBox" x1="1" y1="0" x2="0" y2="1">
            <stop offset="0%" stop-color="#00ff00"/>   
            <stop offset="100%" stop-color="#00ffff"/>   
        </linearGradient>
        <linearGradient id="cyablu" gradientUnits="objectBoundingBox" x1="1" y1="1" x2="0" y2="0">
            <stop offset="0%" stop-color="#00ffff"/>   
            <stop offset="100%" stop-color="#0000ff"/>   
        </linearGradient>
        <linearGradient id="blumag" gradientUnits="objectBoundingBox" x1="0" y1="1" x2="0" y2="0">
            <stop offset="0%" stop-color="#0000ff"/>   
            <stop offset="100%" stop-color="#ff00ff"/>   
        </linearGradient>
        <linearGradient id="magred" gradientUnits="objectBoundingBox" x1="0" y1="1" x2="1" y2="0">
            <stop offset="0%" stop-color="#ff00ff"/>   
            <stop offset="100%" stop-color="#ff0000"/>   
        </linearGradient>
      </defs>
    
      <g fill="none" stroke-width="15" transform="translate(100,100)">
        <path d="M 0,-100 A 100,100 0 0,1 86.6,-50" stroke="url(#redyel)"/>
        <path d="M 86.6,-50 A 100,100 0 0,1 86.6,50" stroke="url(#yelgre)"/>
        <path d="M 86.6,50 A 100,100 0 0,1 0,100" stroke="url(#grecya)"/>
        <path d="M 0,100 A 100,100 0 0,1 -86.6,50" stroke="url(#cyablu)"/>
        <path d="M -86.6,50 A 100,100 0 0,1 -86.6,-50" stroke="url(#blumag)"/>
        <path d="M -86.6,-50 A 100,100 0 0,1 0,-100" stroke="url(#magred)"/>
      </g>
    </svg>

Fiddle here: http://jsfiddle.net/Weytu/

Update 2:

For those that want more than six segments, here is some javascript that will produce a wheel with any number of segments that you wish.

function makeColourWheel(numSegments)
{
    if (numSegments <= 0)
        numSegments = 6;
    if (numSegments > 360)
        numSegments = 360;

    var  svgns = xmlns="http://www.w3.org/2000/svg";
    var  svg = document.getElementById("colourwheel");
    var  defs = svg.getElementById("defs");
    var  paths = svg.getElementById("paths");

    var  radius = 100;
    var  stepAngle = 2 * Math.PI / numSegments;

    var  lastX = 0;
    var  lastY = -radius;
    var  lastAngle = 0;
    
    for (var i=1; i<=numSegments; i++)
    {
        var  angle = i * stepAngle;

        // Calculate this arc end point
        var x = radius * Math.sin(angle);
        var y = -radius * Math.cos(angle);
        // Create a path element
        var arc = document.createElementNS(svgns, "path");
        arc.setAttribute("d", "M " + lastX.toFixed(3) + "," + lastY.toFixed(3)
                              + " A 100,100 0 0,1 " + x.toFixed(3) + "," + y.toFixed(3));
        arc.setAttribute("stroke", "url(#wheelseg" + i + ")");
        // Append it to our SVG
        paths.appendChild(arc);
        
        // Create a gradient for this segment
        var grad = document.createElementNS(svgns, "linearGradient");
        grad.setAttribute("id", "wheelseg"+i);
        grad.setAttribute("gradientUnits", "userSpaceOnUse");
        grad.setAttribute("x1", lastX.toFixed(3));
        grad.setAttribute("y1", lastY.toFixed(3));
        grad.setAttribute("x2", x.toFixed(3));
        grad.setAttribute("y2", y.toFixed(3));
        // Make the 0% stop for this gradient
        var stop = document.createElementNS(svgns, "stop");
        stop.setAttribute("offset", "0%");
        hue = Math.round(lastAngle * 360 / Math.PI / 2);
        stop.setAttribute("stop-color", "hsl(" + hue + ",100%,50%)");
        grad.appendChild(stop);
        // Make the 100% stop for this gradient
        stop = document.createElementNS(svgns, "stop");
        stop.setAttribute("offset", "100%");
        hue = Math.round(angle * 360 / Math.PI / 2);
        stop.setAttribute("stop-color", "hsl(" + hue + ",100%,50%)");
        grad.appendChild(stop);
        // Add the gradient to the SVG
        defs.appendChild(grad);

        // Update lastx/y
        lastX = x;
        lastY = y;
        lastAngle = angle;
    }
}


makeColourWheel(60);

<svg id="colourwheel" xmlns="http://www.w3.org/2000/svg" version="1.1" width="100%" height="100%" viewBox="-10 -10 220 220">
  <defs id="defs">
  </defs>

  <g id="paths" fill="none" stroke-width="15" transform="translate(100,100)">
  </g>
</svg>