score:1

Accepted answer

You are parsing your values using only the abbreviated month name. If you don't specify a year, it defaults to 1900.

Let's proof that:

console.log("A date without year: " + d3.timeParse("%b")("Jan"))
<script src="https://d3js.org/d3.v4.min.js"></script>

So, that 1900 is expected.

Solution: if you want to make sure that the axis only shows the full month name, use tickFormat with the correct specifier:

.tickFormat(function(d){
    return d3.timeFormat("%B")(d)
})

Here is your updated code: 

text {
  font: 10px sans-serif;
}

.axis path,
.axis line {
  fill: none;
  stroke: #000;
  shape-rendering: crispEdges;
}

.line {
  fill: none;
  stroke-width: 1.5px;
}

.label {
  text-anchor: middle;
}

.label rect {
  fill: white;
}

.label-key {
  font-weight: bold;
}
<svg width="960" height="500"></svg>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script>
  var parseTime = d3.timeParse("%b");

  var svg = d3.select("svg");

  var margin = {
      top: 30,
      right: 50,
      bottom: 30,
      left: 30
    },
    width = +svg.attr("width") - margin.left - margin.right,
    height = +svg.attr("height") - margin.top - margin.bottom,
    labelPadding = 3;

  var g = svg.append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

  var data = [{
    date: 'Jan',
    2017: 130,
    2016: 40,
    2015: 140
  }, {
    date: 'Feb',
    2017: 137,
    2016: 58,
    2015: 120
  }, {
    date: 'Mar',
    2017: 166,
    2016: 97,
    2015: 140
  }, {
    date: 'Apr',
    2017: 154,
    2016: 117,
    2015: 40
  }, {
    date: 'May',
    2017: 179,
    2016: 98,
    2015: 110
  }, {
    date: 'Jun',
    2017: 187,
    2016: 120,
    2015: 140
  }, {
    date: 'Jul',
    2017: 189,
    2016: 84,
    2015: 30
  }, {
    date: 'Aug',
    2017: 137,
    2016: 58,
    2015: 50
  }, {
    date: 'Sep',
    2017: 166,
    2016: 97,
    2015: 55
  }, {
    date: 'Oct',
    2017: 154,
    2016: 117,
    2015: 80
  }, {
    date: 'Nov',
    2017: 179,
    2016: 98,
    2015: 40
  }, {
    date: 'Dec',
    2017: 187,
    2016: 120,
    2015: 140
  }];

  data.columns = ["date", "2017", "2016", "2015"]


  data.forEach(d => {
    d.date = parseTime(d.date);
    for (var k in d)
      if (k !== "da") d[k] = +d[k];
  });



  var series = data.columns.slice(1).map(function(key) {
    return data.map(function(d) {
      return {
        key: key,
        date: d.date,
        value: d[key]
      };
    });
  });

  var x = d3.scaleTime()
    .domain([data[0].date, data[data.length - 1].date])
    .range([0, width]);

  var y = d3.scaleLinear()
    .domain([0, d3.max(series, function(s) {
      return d3.max(s, function(d) {
        return d.value;
      });
    })])
    .range([height, 0]);

  var z = d3.scaleOrdinal(d3.schemeCategory10);

  g.append("g")
    .attr("class", "axis axis--x")
    .attr("transform", "translate(0," + height + ")")
    .call(d3.axisBottom(x)
      .tickFormat(function(d) {
        return d3.timeFormat("%B")(d)
      }));

  var serie = g.selectAll(".serie")
    .data(series)
    .enter().append("g")
    .attr("class", "serie");

  serie.append("path")
    .attr("class", "line")
    .style("stroke", function(d) {
      return z(d[0].key);
    })
    .attr("d", d3.line()
      .x(function(d) {
        return x(d.date);
      })
      .y(function(d) {
        return y(d.value);
      }));

  var label = serie.selectAll(".label")
    .data(function(d) {
      return d;
    })
    .enter().append("g")
    .attr("class", "label")
    .attr("transform", function(d, i) {
      return "translate(" + x(d.date) + "," + y(d.value) + ")";
    });

  label.append("text")
    .attr("dy", ".35em")
    .text(function(d) {
      return d.value;
    })
    .filter(function(d, i) {
      return i === data.length - 1;
    })
    .append("tspan")
    .attr("class", "label-key")
    .text(function(d) {
      return " " + d.key;
    });

  label.insert("rect", "text")
    .datum(function() {
      return this.nextSibling.getBBox();
    })
    .attr("x", function(d) {
      return d.x - labelPadding;
    })
    .attr("y", function(d) {
      return d.y - labelPadding;
    })
    .attr("width", function(d) {
      return d.width + 2 * labelPadding;
    })
    .attr("height", function(d) {
      return d.height + 2 * labelPadding;
    });

</script>


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