Sankey diagrams are a specific type of flow diagram typically used to visualise the flow of material, energy, cost, or any measurable resource, shown proportionally to the flow quantity. They draw the attention of the reader to the largest flows, the largest consumer, the main losses, etc. Supported by different colours, flow quantities that have different dimensions are understood intuitively.

Sankey diagrams use links with width proportional to the flow quantity visualized-- if a flow is twice as wide, it represents double the quantity. A Sankey diagram has multiple nodes which are connected by a link. Each node should only appear once and there can be utmost one link between a pair of nodes. They are widely used in energy efficiency, marketing analysis, supply chain management, production systems and many other industries.

When and Why Should You Use the Sankey Diagram

  • Sankey Diagrams are effective to explain complex processes with the spotlight on a single aspect that you want to focus. Say, your team is deciding about energy efficiency. A Sankey diagram can break down this complex process into a visually simple diagram.

  • It is possible to make significant flows/ factors stand out with the Sankey diagram. This helps your audience see regions with the largest opportunities.

  • It is also possible to detect data inconsistencies as easily.

Disadvantages of Sankey diagrams- When they are not appropriate

  • If your audience is new to data visualization, Sankey diagrams are hard to digest for them.

  • Sankey diagrams can make it difficult to differentiate when flows have similar widths.

  • If the nodes are not spaced out well, the Sankey diagram cannot highlight an actionable insight, because of the link overlapping and could result in cluttering.

  • Sankey diagrams almost always require a lengthy introduction and explanation, since not everyone is familiar with this diagram.

Questions to consider before starting your Sankey Diagram:

  • Are you using this Sankey diagram to maximize insight into a data set and for other exploratory data analysis (EDA)?

  • Are you using it to get the audience to perform a particular action, or to tell a story?

  • Does your audience have considerable data visualization experience? Will they be comfortable with an advanced data visualization diagram?

  • Each visualisation should bring out some fact evidently. What would be this fact in your data: maximum contribution? Or flow relationship between relatable entities?

As mentioned earlier, Sankey diagrams are a great alternative to common flow chart or bar & pie charts. While bar and pie charts help with comparison in general, Sankey chart helps compare the flow between relatable entities.

Some of the charts related to it are as below.

  • Parallel sets Parallel Set charts are similar to Sankey Diagrams in the way they show flow and proportions. However, Parallel Sets don’t use arrows and they divide the flow-path at each displayed line-set.

    Each line-set corresponds to a dimension/dataset, which its values/categories are represented in each line divide in that line-set. The width of each line and the flow-path that stems from it is determined by the proportional fraction of the category total. Each flow-path can be coloured to show and compare the distribution between different categories.

  • Alluvial diagram Alluvial diagrams are a type of flow diagram originally developed to represent changes in network structure over time.

Use cases and industry applications of the Sankey Diagram

Sankey Diagram is often used in energy management, manufacturing or in fields of science because of the ease of visualizing the flow of energy and materials in different processes.

Sankey diagrams are suitable for a wide range of applications including:

  • Energy management systems: to understand the energy losses of a particular machine

  • Material flow analyses: to visualize material flows within specific economic sectors

  • Production systems: production cost optimization by understanding the process flow

  • supply chain management: improving operational efficiency to run a more sustainable business operation.

  • Marketing analysis: cash flow analysis of ROI of marketing campaigns

  • Chemical & petroleum engineering: represent energy inputs, useful output, and wasted output

Utility of the Sankey diagram:


Horizontal Sankey diagram depicts the flow of material, with a horizontal orientation

Vertical Sankey diagram depicts the flow of material substantially, with vertical orientation

The Power of the Sankey Diagram in Data Viz

The core of data visualization is to simplify the complexity in understanding the relationship between the data points. Sankey diagram is a very effective data visualization tool. Of course, while the Sankey Diagram can simplify the visualization of the data, the user's understanding and experience with DV who is consuming this information also matters, because of the complexity of the chart itself.

Always remember…

...that the width of the lines and arrows used in the Sankey diagram should appear representative of the amounts/volumes. If something appears disproportionate, contact the diagram maker.