drawing a line on linegraph on mouseover?

Question

I am building a line graph with D3.js. When the user mouses over the graph, I would like to draw a vertical line on the graph, highlight its intersection with the chart lines, and show a tooltip, like this screenshot:

I have got part of the way down this road. Here is my code on JSFiddle: http://jsfiddle.net/BvuBV/1/

As you can see, I am listening for mouse events on the svg element, and the line is (sort of) displaying correctly:

// Add mouseover events.
svg.on("mouseover", function() { 
  console.log('mouseover')
}).on("mousemove", function() {
  console.log('mousemove', d3.mouse(this));
  var x = d3.mouse(this)[0];
  hoverLine.attr("x1", x).attr("x2", x).style("opacity", 1);
}).on("mouseout", function() {
  console.log('mouseout');
  hoverLine.style("opacity", 1e-6);
});

However, there are a couple of problems:

1. The mouseover and mousemove events don't seem to fire consistently when I mouse over the SVG element, only sometimes - am I doing something wrong?
2. I don't know how to translate the x and y values provided by d3.mouse(this) back into date and inlet values, so that I can draw the desired circles on the graph, and show the desired popup.

Any help is very much appreciated.

UPDATE: Thanks to help from @Aegis I have solved 1 and part of 2: http://jsfiddle.net/BvuBV/4/

But I still don't know how to highlight the intersection of the hover line with the two chart lines, and how to retrieve the values of the chart lines at that point.

Answer 1

I know this has already been marked as answered, but for future googlers for whom this shows up as a top result... A working example for a vertical line on mouseover can be found here: http://bl.ocks.org/WillTurman/4631136

Here's the relevant portion of the code for the vertical line:

  var vertical = d3.select(".chart")
        .append("div")
        .attr("class", "remove")
        .style("position", "absolute")
        .style("z-index", "19")
        .style("width", "1px")
        .style("height", "380px")
        .style("top", "10px")
        .style("bottom", "30px")
        .style("left", "0px")
        .style("background", "#fff");

  d3.select(".chart")
      .on("mousemove", function(){  
         mousex = d3.mouse(this);
         mousex = mousex[0] + 5;
         vertical.style("left", mousex + "px" )})
      .on("mouseover", function(){  
         mousex = d3.mouse(this);
         mousex = mousex[0] + 5;
         vertical.style("left", mousex + "px")});

Answer 2

This might help you to get a bit further (I have no experience with D3 at all):

1. For some reason your events are bound to the paths, not the base svg element. If you do d3.select(".air-quality").on(..) it seems to work as you would expect.