Graphs explained

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HEALTH DATA INSTITUTE

What is a graph?

It is important to realise that those things with axes and squiggly lines are NOT graphs: They are charts or plots. In mathematics, graphs describe the surface of 3D objects (topology). When we consider a dodecahedron for example, we can see edges and we can see vertices where the edges intersect - see Figure 1. Leonhard Euler (1707-1783) is considered to be the father of topology.

Figure 1. Describing a dodecahedron

Again in mathematics, it becomes obvious why networks are called graphs. This is what happens when the dodecahedron is removed - see Figure 2.

Figure 2. Vertices and edges form networks

In the era of data science, the vertices are often called nodes and the connections between them are relations. - see Figure 3.

Figure 3. Data science terms for edges and vertices

In a graph data structure, data is stored in labels and properties associated with a node. The power of the structure is labels and properties can also be store in relations. - see Figure 4.

Figure 4. Storing data in a graph structure

For a comparison of relational databases versus graphs go to 'Graphs and relational databases compared'.

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