Why does my saved D3 selection have no effect in s

Question

I'm confused about how to save a D3 selection for later use. In the code below, I have a "global" variable for my axes, to which I save them when they are first created. Later, I'm able to use this variable for certain things (here, setting some text) but not others (here, updating the scale), which only work if I explicitly (re)select the axes.

Is there perhaps something about JavaScript variable scoping or lifetime that I don't understand? Any assistance is appreciated!

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The relevant code, much condensed from the full context:

// Top level of page

var gxaxis, gyaxis;

var updatePlot = function (view, first) {

    d3.csv("data.csv", function (error, data) {
        data.forEach(function (d) {
            ...
        });

        var x, y;
        x = d3.scale.linear().range(...);
        y = d3.scale.linear().range(...);

        x.domain(d3.extent(data, function (d) {...})).nice();
        y.domain(d3.extent(data, function (d) {...})).nice();

        var xAxis = d3.svg.axis().scale(x).orient("bottom");
        var yAxis = d3.svg.axis().scale(y).orient("left");

        // The variable 'first' is true on page load, so this code the if clause will be executed before any executions of the else clause
       if (first) {
            gxaxis = svg.append("g").attr("class", "x axis")
                    .attr("transform", "translate(0," + height + ")")
                    .call(xAxis)
                    .append("text")
                    .attr("class", "label")
                    .attr("x", width)
                    .attr("y", -6)
                    .style("text-anchor", "end");
            gyaxis = svg.append("g").attr("class", "y axis")
                    .call(yAxis)
                    .append("text")
                    .attr("class", "label")
                    .attr("transform", "rotate(-90)")
                    .attr("y", 6)
                    .attr("dy", ".71em")
                    .style("text-anchor", "end");
        } else {
            // Why is it necessary to explicitly select; why can't I use my gxaxis variable (as I do below for the text)?
            svg.select(".x.axis")
                    .transition().duration(0).ease("in")
                    .call(xAxis);
            // Using gyaxis.transition()... has NO EFFECT
            svg.select(".y.axis")
                    .transition().duration(0).ease("in")
                    .call(yAxis);

        }

        var xLabel = ...;
        var yLabel = ...;

        // This use of gxaxis works.
        gxaxis.text(xLabel);
        gyaxis.text(yLabel);

    });

};

// Set up handlers for switching among views
d3.selectAll(".view-select").on("click", function () {
    d3.event.preventDefault();
    updatePlot(this.id, false);
});

updatePlot('a', true);

Answer 1

This code:

gxaxis = svg.append("g").attr("class", "x axis")
                .attr("transform", "translate(0," + height + ")")
                .call(xAxis)
                .append("text")
                .attr("class", "label")
                .attr("x", width)
                .attr("y", -6)
                .style("text-anchor", "end");

Creates this DOM:

<svg>
    <...>
        <g class="x axis">
            <g class="tick">
                <line ...>
                <text ...>
            </g>
        </g>
    </...>
</svg>

And saves the text to the selection, because it was appended last.
If you want the axis, save the axis to the selection before you edit the text.

gxaxis = svg.append("g").attr("class", "x axis")
                .attr("transform", "translate(0," + height + ")")
                .call(xAxis);
gxaxis.selectAll("text")
                .attr("class", "label")
                .attr("x", width)
                .attr("y", -6)
                .style("text-anchor", "end");