22.on-mouse-over-out-transition

<!DOCTYPE html>
<html lang="en">
	<head>
		<meta charset="utf-8">
		<title>D3: Smoother highlight transitions</title>
    
		<script type="text/javascript" src="https://d3js.org/d3.v4.min.js"></script>
		
    <style type="text/css">
			/* No style rules here yet */		
		</style>
    
	</head>
	<body>
		<script type="text/javascript">

var w = 600;
			var h = 250;
			
  
			var svg = d3.select("body")
						.append("svg")
						.attr("width", w)
						.attr("height", h);

var dataset = [ 5, 10, 13, 19, 21, 25, 22, 18, 15, 13,
							11, 12, 15, 20, 18, 17, 16, 18, 23, 25 ];

var xScale = d3.scaleBand()
							.domain(d3.range(dataset.length))
							.rangeRound([0, w])
      				.padding(0.05);

var yScale = d3.scaleLinear()
							.domain([0, d3.max(dataset)])
							.range([h, 0]);

svg.selectAll("rect")
			   .data(dataset)
			   .enter()
			   .append("rect")
			   .attr("x", function(d, i) {
			   		return xScale(i);
			   })
			   .attr("y", function(d) {
			   		return yScale(d);
			   })
			   .attr("width", xScale.bandwidth())
			   .attr("height", function(d) {
			   		return h - yScale(d);
			   })
			   .attr("fill", function(d) {
					return "rgb(0, 0, " + (d * 10) + ")";
			   })
      
// ---- event listener: .on("mouseover", function(){})      
			   .on("mouseover", function() {
			   		d3.select(this)
			   			.attr("fill", "orange");
			   })
      
// ---- event listener: .on("mouseout", function(){})          
			   .on("mouseout", function(d) {
				   d3.select(this)
				   		.transition()
				   		.duration(250)
						.attr("fill", "rgb(0, 0, " + (d * 10) + ")");
			   });

svg.selectAll("text")
			   .data(dataset)
			   .enter()
			   .append("text")
			   .text(function(d) {
			   		return d;
			   })

// ---- on text, pointer events are such as click, mouseover, or mouseout, won't trigger any event
      	 .style("pointer-events", "none")
			   .attr("text-anchor", "middle")
			   .attr("x", function(d, i) {
			   		return xScale(i) + xScale.bandwidth() / 2;
			   })
			   .attr("y", function(d) {
			   		return yScale(d) + 14;
			   })
			   .attr("font-family", "sans-serif")
			   .attr("font-size", "11px")
			   .attr("fill", "white");
			
		</script>
	</body>
</html>

https://d3js.org/d3.v4.min.js