Graphics Reference
In-Depth Information
When traditional approaches are not working to solve a particular problem,
it is useful to step back and re-evaluate what information is truly essential.
What's of interest in this data is not the flow of goods through states, but the
flow of goods in and out of a state from each other state. The few insights
you were able to take away in the previous exercise were most likely spotted
by looking at the direction and thickness of incoming and outgoing lines
directly around a node.
Those patterns are easiest to see, especially if the node lies on the perimeter
whereitislesslikelythatinterferinglineswillcrossit.Iftheinterferinglines
are removed, leaving only the ink that describes incoming and outgoing flow
at each node, it would reveal key characteristics more clearly, no matter
where the nodes lie. That is precisely what a link rose is useful for.
In a link rose, the volume of link flow associated with each node is
summarized by subdividing the space around the node into radial sectors
and aggregating the flow of links for each sector. Flow in and out can be
represented separately, which is useful for a directed graph like this one, or
as a single sum flow.
Figure 13-6 shows the same interstate transport graph using link rose
summaries. The faded inside sectors represent goods entering state borders
from various directions, and the outside sectors represent goods leaving.
Large net goods producers like California, Michigan, and New Jersey are
clearly visible here, as are net consumers like Florida. However, unlike in
the graph representation produced with Gephi, so are smaller net producers
like Minnesota (MN), Oregon (OR), and Massachusetts (MA), as well as net
consumers such as Virginia (VA).
The link rose visualization provides a clearer view of flow of goods to and
from interior and coastal states alike. One limitation, however, is that for
states that have flow pointed in the direction of many other states (such as
California or Massachusetts), it is difficult to tell how far the goods travel.
For example, how much of the flow out the bottom of California heads to
Texas versus Florida? You can get a sense of the relative proportion in that
case by looking at the matching incoming sector in the other two states. But
what about Massachusetts? Interactions could be added to show links when
hovering over a state. However, there is a way to add more information to
the overview as well.
 
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