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2D Histogram

2D Histograms or Density Heatmaps

A 2D histogram, also known as a density heatmap, is the 2-dimensional generalization of a histogram which resembles a heatmap but is computed by grouping a set of points specified by their x and y coordinates into bins, and applying an aggregation function such as count or sum (if z is provided) to compute the color of the tile representing the bin. This kind of visualization (and the related 2D histogram contour, or density contour) is often used to manage over-plotting, or situations where showing large data sets as scatter plots would result in points overlapping each other and hiding patterns. For data sets of more than a few thousand points, a better approach than the ones listed here would be to use Plotly with Datashader to precompute the aggregations before displaying the data with Plotly.

Density Heatmaps with Plotly Express

Plotly Express is the easy-to-use, high-level interface to Plotly, which operates on a variety of types of data and produces easy-to-style figures. The Plotly Express function density_heatmap() can be used to produce density heatmaps.

import plotly.express as px
df = px.data.tips()

fig = px.density_heatmap(df, x="total_bill", y="tip")
fig.show()

The number of bins can be controlled with nbinsx and nbinsy and the color scale with color_continuous_scale.

import plotly.express as px
df = px.data.tips()

fig = px.density_heatmap(df, x="total_bill", y="tip", nbinsx=20, nbinsy=20, color_continuous_scale="Viridis")
fig.show()

Marginal plots can be added to visualize the 1-dimensional distributions of the two variables. Here we use a marginal histogram. Other allowable values are violin, box and rug.

import plotly.express as px
df = px.data.tips()

fig = px.density_heatmap(df, x="total_bill", y="tip", marginal_x="histogram", marginal_y="histogram")
fig.show()

Density heatmaps can also be faceted:

import plotly.express as px
df = px.data.tips()

fig = px.density_heatmap(df, x="total_bill", y="tip", facet_row="sex", facet_col="smoker")
fig.show()

Other aggregation functions than count

By passing in a z value and a histfunc, density heatmaps can perform basic aggregation operations. Here we show average Sepal Length grouped by Petal Length and Petal Width for the Iris dataset.

import plotly.express as px
df = px.data.iris()

fig = px.density_heatmap(df, x="petal_length", y="petal_width", z="sepal_length", histfunc="avg")
fig.show()

2D Histograms with Graph Objects

To build this kind of figure using graph objects without using Plotly Express, we can use the go.Histogram2d class.

2D Histogram of a Bivariate Normal Distribution

import plotly.graph_objects as go

import numpy as np
np.random.seed(1)

x = np.random.randn(500)
y = np.random.randn(500)+1

fig = go.Figure(go.Histogram2d(
        x=x,
        y=y
    ))
fig.show()

2D Histogram Binning and Styling Options

import plotly.graph_objects as go

import numpy as np

x = np.random.randn(500)
y = np.random.randn(500)+1

fig = go.Figure(go.Histogram2d(x=x, y=y, histnorm='probability',
        autobinx=False,
        xbins=dict(start=-3, end=3, size=0.1),
        autobiny=False,
        ybins=dict(start=-2.5, end=4, size=0.1),
        colorscale=[[0, 'rgb(12,51,131)'], [0.25, 'rgb(10,136,186)'], [0.5, 'rgb(242,211,56)'], [0.75, 'rgb(242,143,56)'], [1, 'rgb(217,30,30)']]
    ))
fig.show()

Sharing bin settings between 2D Histograms

This example shows how to use bingroup attribute to have a compatible bin settings for both histograms. To define start, end and size value of x-axis and y-axis separately, set ybins and xbins.

import plotly.graph_objects as go
from plotly.subplots import make_subplots

fig = make_subplots(2,2)
fig.add_trace(go.Histogram2d(
    x = [ 1, 2, 2, 3, 4 ],
    y = [ 1, 2, 2, 3, 4 ],
    coloraxis = "coloraxis",
    xbins = {'start':1, 'size':1}), 1,1)
fig.add_trace(go.Histogram2d(
    x = [ 4, 5, 5, 5, 6 ],
    y = [ 4, 5, 5, 5, 6 ],
    coloraxis = "coloraxis",
    ybins = {'start': 3, 'size': 1}),1,2)
fig.add_trace(go.Histogram2d(
    x = [ 1, 2, 2, 3, 4 ],
    y = [ 1, 2, 2, 3, 4 ],
    bingroup = 1,
    coloraxis = "coloraxis",
    xbins = {'start':1, 'size':1}), 2,1)
fig.add_trace(go.Histogram2d(
    x = [ 4, 5, 5, 5, 6 ],
    y = [ 4, 5, 5, 5, 6 ],
    bingroup = 1,
    coloraxis = "coloraxis",
    ybins = {'start': 3, 'size': 1}),2,2)
fig.show()

2D Histogram Overlaid with a Scatter Chart

import plotly.graph_objects as go

import numpy as np

x0 = np.random.randn(100)/5. + 0.5  # 5. enforces float division
y0 = np.random.randn(100)/5. + 0.5
x1 = np.random.rand(50)
y1 = np.random.rand(50) + 1.0

x = np.concatenate([x0, x1])
y = np.concatenate([y0, y1])

fig = go.Figure()

fig.add_trace(go.Scatter(
    x=x0,
    y=y0,
    mode='markers',
    showlegend=False,
    marker=dict(
        symbol='x',
        opacity=0.7,
        color='white',
        size=8,
        line=dict(width=1),
    )
))
fig.add_trace(go.Scatter(
    x=x1,
    y=y1,
    mode='markers',
    showlegend=False,
    marker=dict(
        symbol='circle',
        opacity=0.7,
        color='white',
        size=8,
        line=dict(width=1),
    )
))
fig.add_trace(go.Histogram2d(
    x=x,
    y=y,
    colorscale='YlGnBu',
    zmax=10,
    nbinsx=14,
    nbinsy=14,
    zauto=False,
))

fig.update_layout(
    xaxis=dict( ticks='', showgrid=False, zeroline=False, nticks=20 ),
    yaxis=dict( ticks='', showgrid=False, zeroline=False, nticks=20 ),
    autosize=False,
    height=550,
    width=550,
    hovermode='closest',

)

fig.show()

Reference

See https://plotly.com/python/reference/histogram2d/ for more information and chart attribute options!

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