Question

我是D3的新手，并试图对我的数据上的上一个和下一个值进行移动平均，以便将其平滑。

目前，我使用之前的2个值+当前值。它确实有效但1）我如何使用下一个值，2）如果我想使用前15个和下15个值怎么办？（拥有30个单独的变量用于存储所有变量将是疯狂的）

我已经习惯了传统的javascript，但在D3中如何以这种方式遍历数据却迷失了方向。希望有人能够启发我，谢谢。

请参阅bl.ocks.org上的完整代码： http://bl.ocks.org/jcnesci/7439277

或者只是这里的数据解析代码：

d3.json("by_date_mod.json", function(error, data) {

// Setup each row of data by formatting the Date for X, and by converting to a number for Y.
data = data.rows;
data.forEach(function(d) {
  d.key = parseDate(String(d.key));
  d.value = +d.value;
});

x.domain(d3.extent(data, function(d) { return d.key; }));
y.domain([0, d3.max(data, function(d) { return d.value; })]);

// Setup the moving average calculation.
// Currently is a hacky way of doing it by manually storing and using the previous 3 values for averaging.
// Looking for another way to address previous values so we can make the averaging window much larger (like 15 previous values).
var prevPrevVal = 0;
var prevVal = 0;
var curVal = 0
var movingAverageLine = d3.svg.line()
.x(function(d,i) { return x(d.key); })
.y(function(d,i) {
    if (i == 0) {
        prevPrevVal  = y(d.value);
        prevVal = y(d.value);
        curVal =  y(d.value);
    } else if (i == 1) {
        prevPrevVal = prevVal;
        prevVal = curVal;
        curVal = (prevVal + y(d.value)) / 2.0;
    } else {
        prevPrevVal = prevVal;
        prevVal = curVal;
        curVal = (prevPrevVal + prevVal + y(d.value)) / 3.0;
    }
    return curVal;
})
.interpolate("basis");

// Draw the moving average version of the data, as a line.
graph1.append("path")
  .attr("class", "average")
  .attr("d", movingAverageLine(data));

// Draw the raw data as an area.
graph1.append("path")
    .datum(data)
    .attr("class", "area")
    .attr("d", area);

// Draw the X-axis of the graph.
graph1.append("g")
    .attr("class", "x axis")
    .attr("transform", "translate(0," + height + ")")
    .call(xAxis);

// Draw the Y-axis of the graph.
graph1.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("Value");
});

Answer 1

您需要一个函数来计算移动平均线：

var movingWindowAvg = function (arr, step) {  // Window size = 2 * step + 1
    return arr.map(function (_, idx) { 
        var wnd = arr.slice(idx - step, idx + step + 1); 
        var result = d3.sum(wnd) / wnd.length;

        // Check for isNaN, the javascript way
        result = (result == result) ? result : _;

        return result; 
    });
};

var avgData = movingWindowAvg(avg, 7); // 15 step moving window.

请注意，当无法提取完整的窗口时，此函数在原始数组的边框处稍微捏碎值。

更新：如果结果为NaN，请将结果转换为开头的当前数字。检查result == result is the recommended way of testing for NaNs in Javascript。

Answer 2

如果你真的不需要一个可变大小的窗口，这个累积平均值可能是一个更快的选择，没有切片开销：

    function cumAvg(objects, accessor) {
      return objects.reduce(
          function(avgs, currObj, i) { 
            if (i == 1) {
              return [ accessor(currObj) ];
            } else {
              var lastAvg = avgs[i - 2]; // reduce idxs are 1-based, arrays are 0
              avgs.push( lastAvg + ( (accessor(currObj) - lastAvg) / i) );
              return avgs;
            }
          }
    }

d3.js具有上一个和下一个数据值的移动平均值

2 个答案: