D3 Examples - 7. date-x-axis histogram

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<!DOCTYPE html>
<meta charset="utf-8">
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<script src="//d3js.org/d3.v4.min.js"></script>
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<style>
  
.bar rect  {
  fill: steelblue;
}
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.bar text {
  fill: #fff;
  font: 10px sans-serif;
}
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</style>
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<body>
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<script>
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/* ---- set time format   ------------------------
1. time format function: --
        code: d3.timeParse()
    
2. time input format in file: --  "%m/%d/%Y %H:%M:%S %p"
    meaning: %p => am/pm
    
-----------------------------------------------------------------------------*/ 
var parseDate = d3.timeParse("%m/%d/%Y %H:%M:%S %p"),
    formatCount = d3.format(",.0f");
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var margin = {top: 10, right: 30, bottom: 30, left: 30},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;
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var svg = d3.select("body").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 + ")");
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// ---- date data scale-map to px --------------------------
//   1. assign xScale a scaleTime func
var xScale = d3.scaleTime()
//   2. set input or date range
    .domain([new Date(2015, 0, 1), new Date(2016, 0, 1)])
//   3. set output or px range 
    .rangeRound([0, width]);
// console.log(x.domain());
  
  
var yScale = d3.scaleLinear()
    .range([height, 0]);
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var histogram = d3.histogram()
// histogram func only use date value from each row of dataset 
// .value() is like a row function to d3.histogram() 
// as .row() to d3.tsv()
    .value(function(d) { return d.date; })
    .domain(xScale.domain())
// group data by weeks
    .thresholds(xScale.ticks(d3.timeWeek));
  
// console.log(xScale.ticks(d3.timeWeek));
// console.log(histogram); 
  
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  svg.append("g")
    .attr("class", "axis axis--x")
    .attr("transform", "translate(0," + height + ")")
    .call(d3.axisBottom(xScale));
 
  
  
// apply date format func to date value of each row of data  
function type(d) {
  d.date = parseDate(d.date);
  return d;
}  
  
  
d3.csv("homicides.csv", type, function(error, data) {
  
  if (error) throw error;
 
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// ---- apply data to histogram func -------------
// meaning: regroup all data into different bins 
  var bins = histogram(data);
// console.log(bins);
  
  
  yScale.domain([0, d3.max(bins, function(d) { return d.length; })]);
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  var bar = svg.selectAll(".bar")
      .data(bins)
        .enter().append("g")
      .attr("class", "bar")
      .attr("transform", function(d) { return "translate(" + xScale(d.x0) + "," + yScale(d.length) + ")"; });
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var rects =  bar.append("rect")
      .attr("x", 1) 
      .attr("width", function(d) { return xScale(d.x1) - xScale(d.x0) - 1; })
      .attr("height", function(d) { return height - yScale(d.length); });
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  bar.append("text")
      .attr("dy", ".75em")// trial and error
      .attr("y", 6) // text backdown 6 px
      .attr("x", function(d) { return (xScale(d.x1) - xScale(d.x0)) / 2; }) 
      .attr("text-anchor", "middle")
      .text(function(d) { return formatCount(d.length); });
});
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</script>
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