带时间刻度的D3直方图

Question

I am trying to make a D3 histogram with a time scale.

There are 2 questions already on the subject ( D3 Histogram - Date Based & D3 time and date histogram ), but I am still having problems.

It if fairly simple, I am using the timestamp of the dates for the x scale:

myData = [
    {"date": new Date("1992-05-01 00:00:00"), "value": 10}, 
    {"date": new Date("1992-05-02 00:00:00"), "value": 110}, 
    {"date": new Date("1992-05-02 00:00:00"), "value": 140}, 
];

var margin = {top: 10, right: 30, bottom: 30, left: 30},
    width = 260 - margin.left - margin.right,
    height = 300 - margin.top - margin.bottom;

timeData = (myData.map(function(d) {return d.date.getTime()}));

var x = d3.scale.linear()
        .domain([d3.min(timeData), d3.max(timeData)])
        .range([0, width]);

var y = d3.scale.linear()
        .domain([0, d3.max(myData, function(d) { return d.value; })])
        .range([height, 0]);

var data = d3.layout.histogram()
            .bins(x.ticks(3))
            .value(function(d) {
                return d.date.getTime()
            })
            (myData);

console.log("histogram data:", data);

var xAxis = d3.svg.axis()
    .scale(x)
    .orient("bottom");

var svg = d3.select(".chart")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
  .append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

var bar = svg.selectAll(".bar")
    .data(data)
  .enter().append("g")
    .attr("class", "bar")
    .attr("transform", function(d) { 
        return "translate(" + x(d.x) + "," + y(d.y) + ")"; 
    });

bar.append("rect")
    .attr("x", 1)
    .attr("width", x(data[0].dx) - 1)
    .attr("height", function(d) { 
        return height - y(d.y); 
    });

bar.append("text")
    .attr("dy", ".75em")
    .attr("y", 6)
    .attr("x", x(data[0].dx) / 2)
    .attr("text-anchor", "middle")
    .text(function(d) { return d.value; });

svg.append("g")
    .attr("class", "x axis")
    .attr("transform", "translate(0," + height + ")")
    .call(xAxis);

JSFiddle here

The strange thing is that there is only one bin.

Any help is welcome

Answer 1

See this fiddle: http://jsfiddle.net/henbox/smxnxvgt/3/

I fixed the "Error: Invalid negative value for attribute width" error first, by changing the width calculation on the rect to:

.attr("width", x(data[0].x + data[0].dx) - 1);

because in the data array that the histogram layout generates the x value is a date but the dx value is a number of milliseconds. See @npdoty's comment on this answer . I also added some CSS to make it easier to see what's going on.

However, to answer the question of why only one bin is created: This is related to using:

.bins(x.ticks(3))

when setting the number of bins. Changing to

.bins(3)

(... or more sensibly 2 in this example), you'll see expected behaviour. This question has a bit more on this, but basically because your x-axis values are dates in milliseconds, ticks were being defined rather arbitrarily \\ unexpectedly. You might expect x (the lower bound of the bin) and dx (the width of the bin) to conform nicely to days & hours, but it doesn't

Finally: both bars are very short because the histogram is counting frequency. So the first date appears once, the second date appears in two records. However, your y-axis range is based on the max value , 140. If you want the bars to show something other than frequency (eg you want to sum the value s in each bin that will need a slightly different approach.