Creating a Sankey Diagram

Introduction

In this tutorial, we will learn how to create a Sankey diagram using Python's Matplotlib library. Sankey diagrams are a type of flow diagram that are commonly used to visualize the flow of energy or materials through a system. They are especially useful for analyzing complex systems with multiple inputs and outputs.

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Import the Required Libraries

We will start by importing the libraries required for creating Sankey diagrams using Matplotlib. We will import the Sankey class from the matplotlib.sankey module, and the pyplot module from Matplotlib.

import matplotlib.pyplot as plt
from matplotlib.sankey import Sankey

Define the Data

Next, we will define the data that we want to visualize using the Sankey diagram. In this example, we will use the data from Example 8.6 from Moran and Shapiro's "Fundamentals of Engineering Thermodynamics". The data represents the flow of energy through a Rankine power cycle. We will define the energy flows as a list of values.

Hdot = [260.431, 35.078, 180.794, 221.115, 22.700,
        142.361, 10.193, 10.210, 43.670, 44.312,
        68.631, 10.758, 10.758, 0.017, 0.642,
        232.121, 44.559, 100.613, 132.168]  ## MW

Create the Figure and Axes

We will create a figure object and add a single set of axes to it. We will also set the title of the plot.

fig = plt.figure(figsize=(8, 9))
ax = fig.add_subplot(1, 1, 1, xticks=[], yticks=[],
                     title="Rankine Power Cycle: Example 8.6 from Moran and "
                     "Shapiro\n\x22Fundamentals of Engineering Thermodynamics "
                     "\x22, 6th ed., 2008")

Create the Sankey Diagram

We will create an instance of the Sankey class and pass in the axes object that we created in step 3. We will also set the format of the flow values and the unit of measurement. We will then add the flows to the diagram using the add method. We will specify the color of each flow using the facecolor parameter, and the labels for each flow using the labels parameter. We will also specify the path lengths for each flow using the pathlengths parameter and the orientation of each flow using the orientations parameter. Finally, we will connect the flows using the connect parameter.

sankey = Sankey(ax=ax, format='%.3G', unit=' MW', gap=0.5, scale=1.0/Hdot[0])
sankey.add(patchlabel='\n\nPump 1', rotation=90, facecolor='#37c959',
           flows=[Hdot[13], Hdot[6], -Hdot[7]],
           labels=['Shaft power', '', None],
           pathlengths=[0.4, 0.883, 0.25],
           orientations=[1, -1, 0])
sankey.add(patchlabel='\n\nOpen\nheater', facecolor='#37c959',
           flows=[Hdot[11], Hdot[7], Hdot[4], -Hdot[8]],
           labels=[None, '', None, None],
           pathlengths=[0.25, 0.25, 1.93, 0.25],
           orientations=[1, 0, -1, 0], prior=0, connect=(2, 1))
sankey.add(patchlabel='\n\nPump 2', facecolor='#37c959',
           flows=[Hdot[14], Hdot[8], -Hdot[9]],
           labels=['Shaft power', '', None],
           pathlengths=[0.4, 0.25, 0.25],
           orientations=[1, 0, 0], prior=1, connect=(3, 1))
sankey.add(patchlabel='Closed\nheater', trunklength=2.914, fc='#37c959',
           flows=[Hdot[9], Hdot[1], -Hdot[11], -Hdot[10]],
           pathlengths=[0.25, 1.543, 0.25, 0.25],
           labels=['', '', None, None],
           orientations=[0, -1, 1, -1], prior=2, connect=(2, 0))
sankey.add(patchlabel='Trap', facecolor='#37c959', trunklength=5.102,
           flows=[Hdot[11], -Hdot[12]],
           labels=['\n', None],
           pathlengths=[1.0, 1.01],
           orientations=[1, 1], prior=3, connect=(2, 0))
sankey.add(patchlabel='Steam\ngenerator', facecolor='#ff5555',
           flows=[Hdot[15], Hdot[10], Hdot[2], -Hdot[3], -Hdot[0]],
           labels=['Heat rate', '', '', None, None],
           pathlengths=0.25,
           orientations=[1, 0, -1, -1, -1], prior=3, connect=(3, 1))
sankey.add(patchlabel='\n\n\nTurbine 1', facecolor='#37c959',
           flows=[Hdot[0], -Hdot[16], -Hdot[1], -Hdot[2]],
           labels=['', None, None, None],
           pathlengths=[0.25, 0.153, 1.543, 0.25],
           orientations=[0, 1, -1, -1], prior=5, connect=(4, 0))
sankey.add(patchlabel='\n\n\nReheat', facecolor='#37c959',
           flows=[Hdot[2], -Hdot[2]],
           labels=[None, None],
           pathlengths=[0.725, 0.25],
           orientations=[-1, 0], prior=6, connect=(3, 0))
sankey.add(patchlabel='Turbine 2', trunklength=3.212, facecolor='#37c959',
           flows=[Hdot[3], Hdot[16], -Hdot[5], -Hdot[4], -Hdot[17]],
           labels=[None, 'Shaft power', None, '', 'Shaft power'],
           pathlengths=[0.751, 0.15, 0.25, 1.93, 0.25],
           orientations=[0, -1, 0, -1, 1], prior=6, connect=(1, 1))
sankey.add(patchlabel='Condenser', facecolor='#58b1fa', trunklength=1.764,
           flows=[Hdot[5], -Hdot[18], -Hdot[6]],
           labels=['', 'Heat rate', None],
           pathlengths=[0.45, 0.25, 0.883],
           orientations=[-1, 1, 0], prior=8, connect=(2, 0))

Add Labels and Formatting

We will add labels to the patches in the Sankey diagram using the text attribute of each patch. We will also format the text to be bold and increase the font size.

diagrams = sankey.finish()
for diagram in diagrams:
    diagram.text.set_fontweight('bold')
    diagram.text.set_fontsize('10')
    for text in diagram.texts:
        text.set_fontsize('10')

Show the Plot

Finally, we will show the Sankey diagram using the show method of the pyplot module.

plt.show()

Summary

In this tutorial, we learned how to create a Sankey diagram using Python's Matplotlib library. We used the Sankey class from the matplotlib.sankey module to create the diagram, and we added labels and formatting to the diagram using the text attribute of each patch. Sankey diagrams are a useful tool for visualizing the flow of energy or materials through a complex system.