Histograms show the frequency distribution of continuous data. They are a good choice for when the data is larger than could be plotted on a bar chart and can be used to find underlying trends in data.
Simple Histogram
Histograms require at least one numeric attribute in the data to be specified using the xKey property. Data will be distributed into bins according to the xKey values.
The simplest configuration for a Histogram Series is shown below:
series: [
{
type: 'histogram',
xKey: 'age',
},
],
Bin Count
By default the histogram will split the X domain of the data into around ten regular-width bins, although the exact number of bins generated will vary so that the chart can find round values for the bin boundaries.
The number of bins to aim for can be overridden by setting the binCount property on a histogram series.
Given enough data, charts with more bins are able to more precisely illustrate underlying trends, but are also more sensitive to random noise.
series: [{
type: 'histogram'
xKey: 'age',
binCount: 20,
}]
Irregular Intervals
Rather than specifying the number of bins, for cases where you know exactly which bins you wish to split your X axis values into, it is possible to explicitly give the start and end values for each bin.
This is given using the bins property, and the value should be an array of arrays where each inner array contains the start and end value of a bin.
For histogram charts with irregular bins, it is usual for the area of the bar, rather than its height, to visually represent the value of each bin. In this way the shape of the underlying curve is maintained over irregular intervals. The areaPlot property should be set to true to enable this mode.
In the example below, the data from the race is split into irregular age categories and the areaPlot property has been set to true.
series: [{
type: 'histogram'
xKey: 'age',
areaPlot: true,
bins: [[16, 18], [18, 21], [21, 25], [25, 40]],
}]
Note that if you provide the bins property, you should refrain from including binCount; however, if both properties are present, 'binCount' takes precedence.
XY Histogram
The histograms shown above all contain a single xKey with its frequency plotted on the Y axis. However, it is also possible to provide Y values corresponding to the X values, by specifying both xKey and yKey properties.
When using XY Histograms it is useful to control how bins are aggregated using the aggregation series property. The following sections compare the sum and mean aggregation functions.
Summing Bins
The sum aggregation function is used to sum the values of a column or attribute for each of the bins.
When a yKey is specified, the default behaviour is to plot a total of the yKey values. The kind of aggregation to use is controlled by the series.aggregation property.
series: [{
type: 'histogram'
xKey: 'age',
yKey: 'winnings',
aggregation: 'sum',
}]
Mean Bins
Showing frequencies or summing up the Y values isn't always the best way to visualize your data.
For data that is not evenly distributed in X, but is relatively uniform in Y, a sum plot XY histogram will tend to be dominated by the populations of the X bins.
In the above example you may notice that the prize money distribution very closely follows the age distribution. In such cases, plotting the mean of a bin on the Y axis better illustrates an underlying trend in the data:
series: [{
type: 'histogram'
xKey: 'age',
yKey: 'time',
yName: 'Race time',
aggregation: 'mean',
}]
API Reference
Properties available on the AgHistogramSeriesOptions interface.
For variable width bins, if true the histogram will represent the aggregated yKey values using the area of the bar. Otherwise, the height of the var represents the value as per a normal bar chart. This is useful for keeping an undistorted curve displayed when using variable-width bins.
Set the bins explicitly. The bins need not be of equal width. Note that bins is ignored if binCount is also supplied.
Dictates how the bins are aggregated. If set to 'sum', the value shown for the bins will be the total of the yKey values. If set to 'mean', it will display the average yKey value of the bin.
Primary identifier for the series. This is provided as seriesId in user callbacks to differentiate multiple series. Auto-generated ids are subject to future change without warning, if your callbacks need to vary behaviour by series please supply your own unique id value.
The data to use when rendering the series. If this is not supplied, data must be set on the chart instead.
Configuration for marker and series highlighting when a series or legend item is hovered over.
A human-readable description of the x-values. If supplied, this will be shown in the default tooltip and passed to the tooltip renderer as one of the parameters.
A human-readable description of the y-values. If supplied, this will be shown in the default tooltip and passed to the tooltip renderer as one of the parameters.