Unable to calculate vertical coordinates for tooltip on Y-axis legend in D3 v4

Question

I have created a graph which has a mouseover/tooltop for each legend item in the y-axis. The mouseover creates a tooltip which spans the width of the graph at the height of the y legend item. I have successfully positioned the height and width as well as x-axis location of the tooltip but cannot come up with the right formula to locate the top of the tooltip.

var countryBox = d3.select("body").append("div")
    .attr("class", "countryTip")
    .style("opacity", 0);

...

/*  Build mouseover infobox for each country in y axis legend in 1800 */
h.selectAll(".axis--y .tick text")
    .on("mouseover", function() {
        totSpec = 0;
        var myElement = d3.select(this);
        var countryName = myElement.text();
        /*  determine absolute coordinates for left edge of SVG */
        var matrix = this.getScreenCTM()
            .translate(+this.getAttribute("cx"), +this.getAttribute("cy"));
        h.selectAll("." + countryName.replace(/\s+/g, '_')).each(function(d) {
            totSpec += d.freqEnd;
            totSpec -= d.freqStart;
            sumSpec = sumSpec + d.freqEnd - d.freqStart;
        });
        var availPercent = totSpec / availSpec;

        countryBox.transition()
            .duration(200)
            .style("opacity", .9);
        var yText = getTranslation(d3.select(this.parentNode).attr("transform"));
        countryBox.html(countryName + '</br>' + r(totSpec) + ' MHz assigned out of ' + r(availSpec) + ' MHz available. <br><b>Band occupancy ' + p(availPercent) + '</b>')
            .style("left", (window.pageXOffset + matrix.e) + "px")
            .style("top", (yText[1] - y.bandwidth()/2 - window.pageYOffset) + "px")
            .style("height", y.bandwidth() + "px")
            .style("width", width + "px");
        console.log("yText[1]: "  + yText[1] + " halfBand: " + halfBand + " margin.top: " + margin.top + " window.pageYOffset: " + window.pageYOffset );

    })
    .on("mouseout", function() {
        countryBox.transition()
            .duration(500)
            .style("opacity", 0);
    });

where

getTranslation is a function to compensate for the absence of d3.transform in v4 to

function getTranslation(transform) {
    var g = document.createElementNS("http://www.w3.org/2000/svg", "g");
    g.setAttributeNS(null, "transform", transform);
    var matrix = g.transform.baseVal.consolidate().matrix;
    return [matrix.e, matrix.f];
}

For

.style("top", (yText[1] - y.bandwidth()/2 - window.pageYOffset) + "px")

• yText[1] returns the y coordinates of the y legend item the mouse is passing over, essentially the vertical midpoint of the country band in question.
• y.bandwidth()/2 represents half the vertical height of the band and
• window.pageYOffset compensates for scroll

As far as I can see, that should put the top of the tooltip at the top of the relevant x band. Obviously not though. Is bootstrap a factor? If it comes to pure svg, I find I can draw a circle perfect laid out on each band use yText[1] as the centre point of the circle. Am I mixing up svg and html?

CSS

div.countryTip {
    position: fixed;
    z-index: 99;
    text-align: left;
    font-family: arial;
    font-size: 1em;
    line-height: 1.5;
    padding: 1.5em;
    background: #144667;
    border: 0px;
    color: white;
    border-radius: 3px;
    pointer-events: none;
    -webkit-transform: translate3d(0, -0.5em, 0);
    -moz-transform: translate3d(0, -0.5em, 0);
    transform: translate3d(0, -0.5em, 0);
    -webkit-transition: opacity 0.1s, -webkit-transform 0.1s;
    -moz-transition: opacity 0.1s, -moz-transform 0.1s;
    transition: opacity 0.1s, transform 0.1s;
}

Working site and full source

Update: Added a Plunker

Answer 1

Finally solved this. I was confusing the SVG and HTML coordinate systems. The answer was to use the y element of the matrix coordinate defined by getScreenCTM()

let matrix = this.getScreenCTM()
   .translate(+this.getAttribute('cx') +this.getAttribute('cy'))

with the result being

.style('top', matrix.f - y.bandwidth() / 2 + 8 + 'px')

This post on understanding SVG coordinates and this practical example on SVG tooltips helped me get there in the end.