D3 histogram, number of occurrences of date from time stamp

Newb here. I have a csv with this data:

date, lat, lon
01/01/2015 09:38:14 AM,37.973424,-87.575423
01/01/2015 09:48:27 PM,37.385218,-122.11413
01/01/2015 10:17:34 AM,39.081712,-76.554603
01/02/2015 01:27:17 PM,40.216204,-74.619533

I want to have a histogram where the x axis is by date and the y axis is by number of occurrences.

So Jan 1 would have a column height of 3 and Jan 2 would have a column height of 1. The time of day is irrelevant.

Do I need to parse the dates, or set a time interval of "day" somehow, or create an array? It seems there are two steps: "filter" the data into chunks, and count the chunks, but I'm not sure how to do it.

I found this example, but the dates are already "rounded" nicely to dates, and the data is in the file, not external.

Thanks for any help!

---

All you need to do is iterate through your data and keep a tally of the frequencies. I modified this bar chart example to use your data. You need to use the Time Formatting methods D3 provides.

var formatDate = d3.time.format("%m/%d/%Y %I:%M:%S %p");

// line = "01/01/2015 09:38:14 AM,37.973424,-87.575423"

var parts = line.split(',');
var datetime = formatDate.parse(parts[0]);
var date = formatDate(datetime).split(' ')[0]; // "01/01/2015"

Full example:

var margin = {
    top: 20,
    right: 20,
    bottom: 30,
    left: 40
},
width = 250 - margin.left - margin.right,
    height = 200 - margin.top - margin.bottom;

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");

var yAxis = d3.svg.axis()
    .scale(y)
    .orient("left")
    .ticks(10);

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 + ")");

var csv = [
'01/01/2015 09:38:14 AM,37.973424,-87.575423',
'01/01/2015 09:48:27 PM,37.385218,-122.11413',
'01/01/2015 10:17:34 AM,39.081712,-76.554603',
'01/02/2015 01:27:17 PM,40.216204,-74.619533'
];

var formatDate = d3.time.format("%m/%d/%Y %I:%M:%S %p");

var tally = {};

csv.forEach(function(line) {
    var parts = line.split(',');
    var datetime = formatDate.parse(parts[0]);
    var date = formatDate(datetime).split(' ')[0];
    
    tally[date] = (tally[date]||0) + 1;
});

var data = [];

for (var date in tally) {
    if (tally.hasOwnProperty(date)) {
        data.push({
            date: date,
            frequency: tally[date]
        });
    }
}

x.domain(data.map(function (d) {
    return d.date;
}));
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);

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.date);
})
    .attr("width", x.rangeBand())
    .attr("y", function (d) {
    return y(d.frequency);
})
    .attr("height", function (d) {
    return height - y(d.frequency);
});


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

.bar {
    fill: steelblue;
}
.bar:hover {
    fill: brown;
}
.axis {
    font: 10px sans-serif;
}
.axis path, .axis line {
    fill: none;
    stroke: #000;
    shape-rendering: crispEdges;
}
.x.axis path {
    display: none;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.4.11/d3.min.js"></script>

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