score:0

I usually go with the title coming down the right side:

    svg.append("g")
        .attr("class", "y axis")
        .attr("transform", "translate(" + eval(margin.left-3) + ",0)")
        .call(yAxis)
        .append("text")
        .text("Title")
        .attr("transform","rotate(90)")
        .attr("y",-2)
        .style('fill','black')
        .style("text-anchor","start")

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