d3.js change color and size on line graph dot on mouseover

Question

I made a line graph with d3.js (see the attached image1).

I managed to insert tooltips on graph dots when mouseover. I'd like to change color and size of dots too. I tried in many ways but it seems really difficult. Any help? Here is the piece of code:

  svg.selectAll("dot")    
    .data(data)         
    .enter().append("circle")                               
    .attr("r", 5.5)
    .style("fill", "#fff8ee")    
       .style("opacity", .8)      // set the element opacity
.style("stroke", "#f93")    // set the line colour
 .style("stroke-width", 3.5) 
    .attr("cx", function(d) { return x(d.date); })       
    .attr("cy", function(d) { return y(d.close); })     
    .on("mouseover", function(d) {   

        div.transition()        
            .duration(70)      
            .style("opacity", .7)

             ;      
        div .html(formatTime(d.date) + "<br/>"  + d.close)  
            .style("left", (d3.event.pageX) + "px")     
            .style("top", (d3.event.pageY - 28) + "px");    
        })                  
    .on("mouseout", function(d) {       
        div.transition()        
            .duration(200)      
            .style("opacity", 0);   
    });

Answer 1

Just set color and size in the handlers:

.on("mouseover", function(d) {
  d3.select(this).attr("r", 10).style("fill", "red");
})                  
.on("mouseout", function(d) {
  d3.select(this).attr("r", 5.5).style("fill", "#fff8ee");
});

Answer 2

I don't know why, but while d3.select(this) used to work, it doesn't anymore. I now use d3.select(event.currentTarget).

So, if we consider svg as the graph and all its circles being red by default, we can change the color of the circles to green on mouseover and return the color to red on mouseout like this:

svg.on("mouseover", function(d){
d3.select(event.currentTarget)
.style("fill", "green");
})
.on("mouseout", function(d){
d3.select(event.currentTarget)
.style("fill", "red");
});