Graphics Reference
In-Depth Information
Tip
For
directed graphs
, A-B and B-A are two different edges, and both
are valid. In an
undirected graph
, A-B and B-A are the same edge,
and only one pair should occur in an undirected graph.
The supplementary data set of airports lists the nodes. In this example, the
individual airports (such as ORD and LAX) are the nodes:
Airport, Name, Latitude, Longitude, AvgFlightsPerDay
ORD, O'Hare, 41.94, -87.9, 2409
LGA, LaGuardia, 40.77, -73.87, 423
LAX, Los Angeles, 33.9, -118.40, 1218
...
In the case of this airline data, the graph itself is simply the nodes (airports)
and links (routes between airports). Additional attributes, such as
AvgFlightsPerDay, Distance, or Duration, may not be important to draw the
graph but may be important in the analysis of the graph and achieving the
goal. For example, finding the shortest flight in duration between London,
England, and Auckland, New Zealand, would require both the graph and the
duration associated with each link.
One challenge with links in general is that most graph visualization software
does not handle many links between the same pair of nodes. Using the flight
example, if 15 flights are listed in the flight database from ORD to LGA,
some types of graph visualization software must receive summarized links
(that is, a single link for ORD-LGA with an extra field indicating a count of
15 flights).
Because many airlines may serve the same route, you have multiple links
per route. To consolidate multiple links into a single link, you can use a
pivot table in the spreadsheet to summarize the data for each route, and
then you use this pivot table as the output for the graph data, as shown
in Chapter 7, and the
Flight Stats
spreadsheet in the Supplementary
Materialonthisbook'scompanionwebsite.Or,youcanconsolidatemultiple
links programmatically, as shown in the e-mail example in Chapter 8.
