How to implement tooltip for D3 line chart with data from 2 arrays?

Question

I am trying to implement a D3 chart with multiple lines that shows the sample size at every point as a tooltip when hovering over the chart. I have the data as two arrays but I don't know how to access the second array relative to the position of the mouse on the chart.

How can I access the values in the second array with the correct index? Here is a jsfiddle showing what I need to fix: http://jsfiddle.net/fMXvv/1/ on line 60 of the js, or the div.html method shown below:

    graph.append("svg:path").attr("d", line(data[0])).style("stroke", "black").style("stroke-width", 2)
        .on("mouseover", function (d, i) {
        div.transition()
            .duration(200)
            .style("opacity", .9);
        div.html("n = " + data[0][i])
            .style("left", (d3.event.pageX) + "px")
            .style("top", (d3.event.pageY - 28) + "px");

The difficulty comes from the fact that the chart has more than one series (3 lines), so knowing the x coordinate is not enough.

Answer 1

I used d3.event.pageY to map mouse coordinates using scale:

var toolTipScale = d3.scale.linear().domain([h + 80, 80]).range([0, max_value]);

and then you use:

div.html("n = " + Math.ceil(toolTipScale( d3.event.pageY)) )

To map mouse position correctly it is necessary to add css

body { margin: 0; padding: 0; }

Here is jsfiddle of this - http://jsfiddle.net/cuckovic/sDnC8/

To access elements from second array you can use d3.event.pageX:

var iScale = d3.scale.linear().domain([w + 80, 80]).range([data[0].length, 0]);

and then:

div.html("n = " + data[1][Math.floor(iScale( d3.event.pageX))] )

jsfiddle: http://jsfiddle.net/cuckovic/sDnC8/5/

Answer 2

I had a problem that might have been similar to yours, I needed to select a time based on the mouse position, this is what I did (might give you some ideas):

var timeScale = d3.scale.linear()
    .domain([startTime.getTime(), endTime.getTime()])
    .range([30, r + 10])
    .clamp(true);

axisOverlay.on("mousemove", mousemove)
    .on("touchmove", mousemove);

function mousemove() {
    selectedTime = new Date(timeScale.invert(d3.mouse(this)[1]));
}