JavaScript Charts: Histogram Series
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 continuous 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 regularwidth bins, although the exact number generated will vary so that the chart can find round values for the bin boundaries. This is similar to how giving a number of ticks to an axis does not guarantee that exact number of ticks. 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 specify the number of bins, for cases where you know exactly which bins you wish to split your xaxis 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. In the example below, the data from the race is split into irregular age categories.
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.
series: [{
type: 'histogram'
xKey: 'age',
areaPlot: true,
bins: [[16, 18], [18, 21], [21, 25], [25, 40]]
}]
Note that if you give the bins
property you should not also give binCount
, but if both are present bins
takes precedence.
XY Histogram
The histograms shown above all contain a single xKey
with its frequency plotted on the y axis. However it is sometimes useful to visualise an xKey
and yKey
using a Histogram.
When using XY Histograms it is also useful to control how bins are aggregated using the aggregation
series property. The following sections compare the sum
and mean
aggregation functions.
Summing Bins
This is used to show the summing of one column or attribute for each of the bins. When a yKey
is given, 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 yvalues isn't always the best way to show 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 xbins. In the above example you may notice that the prize money distribution very closely follows the age distribution, so that while potentially useful, the chart does not reveal any new trends in the data. In many cases, plotting the mean of a bin on the yaxis better illustrates an underlying trend in the data:
series: [{
type: 'histogram'
xKey: 'age',
yKey: 'time',
yName: 'Average Time',
aggregation: 'total'
}]
API Reference
data Required  The data to use when rendering the series. If this is not supplied, data must be set on the chart instead. 
visible  Whether or not to display the series. Default: true 
showInLegend  Whether or not to include the series in the legend. Default: true 
tooltip  Seriesspecific tooltip configuration. See tooltip for more details about this configuration object. 
xKey Required  The key to use to retrieve xvalues from the data. 
xName  A humanreadable description of the xvalues. 
yKey  The key to use to retrieve yvalues from the data. 
yName  A humanreadable description of the yvalues. 
binCount  The number of bins to try to split the x axis into. Clashes with the bins setting.Default: 10 
bins  Set the bins explicitly. The bins need not be of equal width. Clashes with the binCount setting. 
aggregation  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 Default: 'sum' Options: 'sum' , 'mean' 
areaPlot  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 variablewidth binsDefault: 'false' 
lineDash  Defines how the column strokes are rendered. Every number in the array specifies the length in pixels of alternating dashes and gaps. For example, [6, 3] means dashes with a length of 6 pixels with gaps between of 3 pixels.Default: [] 
lineDashOffset  The initial offset of the dashed line in pixels. Default: 0 
highlightStyle  Configuration for the highlighting used when the bars are hovered over. See highlightStyle for more details about this configuration object. 
fill  The colour of the fill for the histogram bars. Default: '#f3622d' 
fillOpacity  The opacity of the fill for the histogram bars. Default: 1 
stroke  The colour of the stroke for the histogram bars. Default: '#aa4520' 
strokeOpacity  The opacity of the stroke for the histogram bars. Default: 1 
strokeWidth  The width in pixels of the stroke for the histogram bars. Default: 1 
listeners  A map of event names to event listeners. See listeners for more details about this configuration object. 
tooltip
Seriesspecific tooltip configuration.
histogram: {
...
tooltip: {
enabled: boolean; // default: true
renderer: Function;
}
}
enabled  Whether or not to show tooltips when the series are hovered over. Default: true 
renderer  Function used to create the content for tooltips.

highlightStyle
Configuration for the highlighting used when the bars are hovered over.
histogram: {
...
highlightStyle: {
fill: string; // default: 'yellow'
stroke: string;
}
}
fill  The fill colour of the bars when hovered over. Default: 'yellow' 
stroke  The colour of the stroke around the bars when hovered over. 
listeners
A map of event names to event listeners.
histogram: {
...
listeners: {
nodeClick: Function;
}
}
nodeClick  The listener to call when a histogram bar is clicked.

Next Up
Continue to the next section to learn about area series.