Label Graph Nodes and Edges

This example shows how to add and customize labels on graph nodes and edges.

Create and Plot Graph

Create a graph representing the gridded streets and intersections in a city. Add weights to the edges so that the main avenues and cross streets appear differently in the plot. Plot the graph with the edge line widths proportional to the weight of the edge.

s = [1 1 2 2 3 4 4 5 5 6 7 7 8 8 9 10 11];
t = [2 4 3 5 6 5 7 6 8 9 8 10 9 11 12 11 12];
weights = [1 5 1 5 5 1 5 1 5 5 1 5 1 5 5 1 1];
G = graph(s,t,weights);
P = plot(G,'LineWidth',G.Edges.Weight);

Add Node Labels

For graphs with 100 or fewer nodes, MATLAB® automatically labels the nodes using the numeric node indices or node names (larger graphs omit these labels by default). However, you can change the node labels by adjusting the NodeLabel property of the GraphPlot object P or by using the labelnode function. Therefore, even if the nodes have names, you can use labels that are different from the names.

Remove the default numeric node labels. Label one of the intersections as Home and another as Work.

labelnode(P,1:12,'')
labelnode(P,5,'Home')
labelnode(P,12,'Work')

Add Edge Labels

The edges in a plotted graph are not labeled automatically. You can add edge labels by changing the value of the EdgeLabel property of the GraphPlot object P or by using the labeledge function.

Add edge labels for streets in New York City. The order of the edges is defined in the G.Edges table of the graph, so the order of the labels you specify must respect that order. It is convenient to store edge labels directly in the G.Edges table, so that the edge name lives right next to the other edge information.

G.Edges

ans=17×2 table
    EndNodes    Weight
    ________    ______

     1     2      1   
     1     4      5   
     2     3      1   
     2     5      5   
     3     6      5   
     4     5      1   
     4     7      5   
     5     6      1   
     5     8      5   
     6     9      5   
     7     8      1   
     7    10      5   
     8     9      1   
     8    11      5   
     9    12      5   
    10    11      1   
      ⋮

This example has 17 edges but only 7 unique street names. Therefore, it makes sense to define the street names in a cell array and then index into the cell array to retrieve the desired street name for each edge. Add a variable to the G.Edges table containing the street names.

streets = {'8th Ave' '7th Ave' '6th Ave' '5th Ave' ...
    'W 20th St' 'W 21st St' 'W 22nd St'}';
inds = [1 5 1 6 7 2 5 2 6 7 3 5 3 6 7 4 4];
G.Edges.StreetName = streets(inds);
G.Edges

ans=17×3 table
    EndNodes    Weight     StreetName  
    ________    ______    _____________

     1     2      1       {'8th Ave'  }
     1     4      5       {'W 20th St'}
     2     3      1       {'8th Ave'  }
     2     5      5       {'W 21st St'}
     3     6      5       {'W 22nd St'}
     4     5      1       {'7th Ave'  }
     4     7      5       {'W 20th St'}
     5     6      1       {'7th Ave'  }
     5     8      5       {'W 21st St'}
     6     9      5       {'W 22nd St'}
     7     8      1       {'6th Ave'  }
     7    10      5       {'W 20th St'}
     8     9      1       {'6th Ave'  }
     8    11      5       {'W 21st St'}
     9    12      5       {'W 22nd St'}
    10    11      1       {'5th Ave'  }
      ⋮

Update the EdgeLabel property to reference these street names.

P.EdgeLabel = G.Edges.StreetName;

Adjust Font Properties

The node and edge labels in a graph plot have their own properties that control the appearance and style of the labels. Since the properties are decoupled, you can use different styles for the node labels and the edge labels.

For Node labels, you can adjust:

• NodeLabel

• NodeLabelColor

• NodeFontName

• NodeFontSize

• NodeFontWeight

• NodeFontAngle

For Edge labels, you can adjust:

• EdgeLabel

• EdgeLabelColor

• EdgeFontName

• EdgeFontSize

• EdgeFontWeight

• EdgeFontAngle

Use these properties to adjust the fonts in this example with New York City Streets:

• Change NodeFontSize and NodeLabelColor so that the intersection labels are 12 pt. font and red.

• Change EdgeFontWeight, EdgeFontAngle, and EdgeFontSize to use a larger, bold font for streets in one direction and a smaller, italic font for streets in the other direction.

• Change EdgeFontName to use Times New Roman for the edge labels.

You can use the highlight function to change the graph properties of a subset of the graph edges. Create a logical index isAvenue that is true for edge labels containing the word 'Ave'. Using this logical vector as an input to highlight, label all of the Avenues in one way, and all of the non-Avenues another way.

P.NodeFontSize = 12;
P.NodeLabelColor = 'r';
isAvenue = contains(P.EdgeLabel, 'Ave');
highlight(P, 'Edges', isAvenue, 'EdgeFontAngle', 'italic', 'EdgeFontSize', 7);
highlight(P, 'Edges', ~isAvenue, 'EdgeFontWeight', 'bold', 'EdgeFontSize', 10);
P.EdgeFontName = 'Times New Roman';

Highlight Edges

Find the shortest path between the Home and Work nodes and examine which streets are on the path. Highlight the nodes and edges on the path in red and remove the edge labels for all edges that are not on the path.

[path,d,pathEdges] = shortestpath(G,5,12)

path = 1×4

     5     6     9    12

d = 
11

pathEdges = 1×3

     8    10    15

G.Edges.StreetName(pathEdges,:)

ans = 3x1 cell
    {'7th Ave'  }
    {'W 22nd St'}
    {'W 22nd St'}

highlight(P,'Edges',pathEdges,'EdgeColor','r')
highlight(P,path,'NodeColor','r')
labeledge(P, setdiff(1:numedges(G), pathEdges), '')