score:10

Accepted answer

EDITS

For those finding this answer through a search, also see the answer I wrote here. It's a very similar question with a much cleaner implementation.


I didn't look at the implementation of the linked example but recreated the effect. I went a little over the top playing around with this, so before I explain here's the updated fiddle.

Essentially I added a vertical line to follow the mouse. I also added a g of a circle and text element for each plotted line. When you mouse over the plot, I update the vertical line to the mouse position, figure out where the vertical line is on the x-axis and determine where it intersects each plotted path. Finally, I updated the position of the circle and update the text elements with the interpolated x,y positions.

Here's a commented explanation of the added code:

svg.append("path") // this is the black vertical line to follow mouse
  .attr("class","mouseLine")  
  .style("stroke","black")
  .style("stroke-width", "1px")
  .style("opacity", "0");

var mouseCircle = causation.append("g") // for each line, add group to hold text and circle
      .attr("class","mouseCircle"); 

mouseCircle.append("circle") // add a circle to follow along path
  .attr("r", 7)
  .style("stroke", function(d) { console.log(d); return color(d.key); })
  .style("fill","none")
  .style("stroke-width", "1px"); 

mouseCircle.append("text")
  .attr("transform", "translate(10,3)"); // text to hold coordinates

var bisect = d3.bisector(function(d) { return d.YEAR; }).right; // reusable bisect to find points before/after line

svg.append('svg:rect') // append a rect to catch mouse movements on canvas
  .attr('width', width) // can't catch mouse events on a g element
  .attr('height', height)
  .attr('fill', 'none')
  .attr('pointer-events', 'all')
  .on('mouseout', function(){ // on mouse out hide line, circles and text
        d3.select(".mouseLine")
            .style("opacity", "0");
        d3.selectAll(".mouseCircle circle")
            .style("opacity", "0");
      d3.selectAll(".mouseCircle text")
            .style("opacity", "0");
  })
  .on('mouseover', function(){ // on mouse in show line, circles and text
        d3.select(".mouseLine")
            .style("opacity", "1");
         d3.selectAll(".mouseCircle circle")
            .style("opacity", "1");
        d3.selectAll(".mouseCircle text")
            .style("opacity", "1");
  })
  .on('mousemove', function() { // mouse moving over canvas
      d3.select(".mouseLine")
      .attr("d", function(){
          yRange = y.range(); // range of y axis
          var xCoor = d3.mouse(this)[0]; // mouse position in x
          var xDate = x.invert(xCoor); // date corresponding to mouse x 
          d3.selectAll('.mouseCircle') // for each circle group
              .each(function(d,i){
                 var rightIdx = bisect(data[1].values, xDate); // find date in data that right off mouse
                 var interSect = get_line_intersection(xCoor,  // get the intersection of our vertical line and the data line
                      yRange[0], 
                      xCoor, 
                      yRange[1],
                      x(data[i].values[rightIdx-1].YEAR),
                      y(data[i].values[rightIdx-1].VALUE),
                      x(data[i].values[rightIdx].YEAR),
                      y(data[i].values[rightIdx].VALUE));

              d3.select(this) // move the circle to intersection
                  .attr('transform', 'translate(' + interSect.x + ',' + interSect.y + ')');

              d3.select(this.children[1]) // write coordinates out
                  .text(xDate.toLocaleDateString() + "," + y.invert(interSect.y).toFixed(0));

              });

          return "M"+ xCoor +"," + yRange[0] + "L" + xCoor + "," + yRange[1]; // position vertical line
      });
  });

// from here: https://stackoverflow.com/a/1968345/16363
function get_line_intersection(p0_x, p0_y, p1_x, p1_y, 
    p2_x, p2_y, p3_x, p3_y)
{
    var rV = {};
    var s1_x, s1_y, s2_x, s2_y;
    s1_x = p1_x - p0_x;     s1_y = p1_y - p0_y;
    s2_x = p3_x - p2_x;     s2_y = p3_y - p2_y;

    var s, t;
    s = (-s1_y * (p0_x - p2_x) + s1_x * (p0_y - p2_y)) / (-s2_x * s1_y + s1_x * s2_y);
    t = ( s2_x * (p0_y - p2_y) - s2_y * (p0_x - p2_x)) / (-s2_x * s1_y + s1_x * s2_y);

    if (s >= 0 && s <= 1 && t >= 0 && t <= 1)
    {
        // Collision detected
        rV.x = p0_x + (t * s1_x);
        rV.y = p0_y + (t * s1_y);
    }

    return rV;
}

Screenshot:

enter image description here


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