Effecting delayed transitions on d3js mouseout events
<p>Here is an excerpt from my d3.js scatter plot.</p>

<pre><code>svg.on("click", function() {
        d3.select("#tooltip_svg_01").classed("hidden", true);
   });
svg.selectAll("circle")
   .data(dataset)
   .enter()
   .append("circle")
   .attr("cx", function(d) {
        return xScale(d[x']);
   })
   .attr("cy", function(d) {
        return yScale(d['y']);
   })
   .attr("r", scatter_radius)
   .style("stoke", "gray")

   .on("mouseover", function(d) {
        console.log('mouseover: ' + d3.select(this));
        var xPosition = parseFloat(d3.select(this).attr("cx")) + scatter_radius;
        var yPosition = parseFloat(d3.select(this).attr("cy"));
        d3.select("#tooltip_svg_01")
            .style("left", xPosition + "px")
            .style("top", yPosition + "px")                     
            .select("#value_tt_01")
            .text(d['x'] + ',' + d['y']);

        d3.select(this).style("fill", 'red');
        //Show the tooltip
        d3.select("#tooltip_svg_01").classed("hidden", false);
   })
   .on("mouseout", function() {
        console.log('mouseout: ' + d3.select(this));
        d3.select("#tooltip_svg_01").transition().delay(30).attr("class", "hidden");
        d3.select(this).transition().delay(30).style("fill", "gray");
   })
</code></pre>

<p>The attempts in the <code>mouseout</code> event to institutea delay before the tooltip disappears and a delay before the circle turns back are not working. The changes are made immediately.
How can this be done in <code>d3.js</code> ?</p>