d3 - how to add x-axis labels for only years on a monthly chart?

I'm creating a bar chart in d3 (using r2d3 in R) that shows data per month. I'm wondering how I can format the x-axis so that while there is a bar for each month, the labels only show the year markers (i.e. under each january bar.)

Data look like this:

letter,frequency
1/1/2017,144
2/1/2017,85
3/1/2017,59
4/1/2017,73
5/1/2017,68
6/1/2017,91
7/1/2017,107
8/1/2017,94
9/1/2017,79
10/1/2017,84
11/1/2017,70
12/1/2017,86
1/1/2018,72
2/1/2018,71
3/1/2018,82
4/1/2018,50
5/1/2018,86
6/1/2018,75
7/1/2018,62
8/1/2018,72
9/1/2018,65
10/1/2018,75
11/1/2018,63
12/1/2018,87
1/1/2019,59
2/1/2019,60
3/1/2019,99
4/1/2019,81

Data in console:

D3 Code:

var parseDate = d3.time.format("%Y-%m-%d").parse;

//coerce data to dates
data.forEach(function (d) {
  d.letter = parseDate(d.letter);
  console.log(d.letter)
});

//set margins 
var margin = {top: 40, right: 20, bottom: 60, left: 40},
    width = 1000 - margin.left - margin.right,
    height = 200 - margin.top - margin.bottom;

var formatPercent = d3.format(".0%");

var x = d3.scale.ordinal()
    .rangeRoundBands([0, width], .1);
    

var y = d3.scale.linear()
    .range([height, 0]);

var xAxis = d3.svg.axis()
    .scale(x)
    .orient("bottom")
    .tickFormat(d3.time.format("%Y-%m"));

var yAxis = d3.svg.axis()
    .scale(y)
    .orient("left")


var svg = div.append("svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
  .append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");


r2d3.onRender(function(data, s, w, h, options) {
  x.domain(data.map(function(d) { return d.letter; }));
  y.domain([0, d3.max(data, function(d) { return d.frequency; })]);

  svg.append("g")
      .attr("class", "x axis")
      .attr("transform", "translate(0," + height + ")")
      .call(xAxis)
    .selectAll("text")
      .attr("transform", "rotate(270)")
      .attr('dx', '-1.9em');

      
  svg.append("g")
      .attr("class", "y axis")
      .call(yAxis)
    .append("text")
      .attr("transform", "rotate(-90)")
      .attr("y", 6)
      .attr("dy", ".71em")
      .style("text-anchor", "end")
      .text("Frequency");

  svg.selectAll(".bar")
      .data(data)
    .enter().append("rect")
      .attr("class", "bar")
      .attr("x", function(d) { return x(d.letter); })
      .attr("width", x.rangeBand())
      .attr("y", function(d) { return y(d.frequency); })
      .attr("height", function(d) { return height - y(d.frequency); })
      .on('mouseover', tip.show)
      .on('mouseout', tip.hide)

});

function type(d) {
  d.frequency = +d.frequency;
  console.log(d);
  return d;
}

Current Chart:

What I want:

Here's how D3 suggests filtering ordinal scales. This should only show your January ticks

var xAxis = d3.svg.axis()
    .scale(x)
    .orient("bottom")
    .tickValues(x.domain().filter(function(d, i) { return !(i % 12); }))