Home > Articles

The Chord Diagram

  • Print
  • + Share This
This chapter is from the book

Disjointed City Pairs

Airlines are very good at resource optimization. An airplane goes from airport to airport, picking up and delivering passengers at each stop. I wondered, though, how often does an airplane fly an “empty leg” route? By that I mean, how often is the destination airport different from the next origination airport for a given airplane? We have the data that we need to answer that question!

Using the Lead Window Function to Find Empty Leg Flights

The idea is simple enough. We have the airplanes’ tail numbers, so we can track each plane’s movements. We can ask Postgres to tell us the next origin airport using the LEAD window function and compare that to that record’s destination airport. The query to do all of those things requires a few steps, though. I typically use Common Table Expression (CTE) when I have queries with multiple parts that build on each other.

If you wanted to just write out the query and see the data, it would look like this:

WITH base_query AS (
  SELECT id,
    to_date((((year || '-'::text) || month) || '-'::text) || day_of_month, 'YYYY-MM-DD'::text) AS dep_date,
   FROM departures
   WHERE unique_carrier = 'AA'
), departures_with_lead AS (
    LEAD(origin) OVER (PARTITION BY tail_num ORDER BY tail_num, dep_date, dep_time) AS next_origin
  FROM base_query
SELECT * FROM departures_with_lead
WHERE unique_carrier = 'AA'
  AND dest <> next_origin;

Go ahead and open your favorite SQL editor, or go into rails dbconsole, and run that query.

It’s not crazy slow, but it’s also not fast. For that reason, I will optimize and put the base query in a materialized view.

Optimizing Slow Queries with the Materialized View

A view is just a query that is stored in the database. A materialized view is where the query AND the results are stored in the database. You can index and update the data in a materialized view. We won’t need to update this data, but indexes will be handy.

We can use Rails’s migrations to do this for us. Create a migration (rails g migration create_aa_departures). Put the following code in the new migration and run the migration:

class CreateAaDepartures < ActiveRecord::Migration
  def up
    sql = <<-SQL.strip_heredoc
      CREATE MATERIALIZED VIEW aa_departures AS (
        WITH base_query AS (
          SELECT id,
            to_date((((year || '-'::text) || month) || '-'::text) || day_of_month, 'YYYY-MM-DD'::text) AS dep_date,
           FROM departures
           WHERE unique_carrier = 'AA'
        SELECT *,
          LEAD(origin) OVER (PARTITION BY tail_num ORDER BY tail_num, dep_date, dep_time) AS next_origin
        FROM base_query
      CREATE INDEX ON aa_departures USING btree (origin);
      CREATE INDEX ON aa_departures USING btree (dest);
      CREATE INDEX ON aa_departures USING btree (next_origin);

  def down
    execute("DROP MATERIALIZED VIEW aa_departures;")

Draw the Disjointed City Pairs Chord Diagram

The query was the hard part. Now we just need to add the plumbing in the DeparturesController to query the data and serve it to the JavaScript.

The new controller methods should look very familiar:

def disjointed; end
def disjointed_matrix
  airports, matrix = Departure.disjointed_matrix
  render :json => {
    :airports => airports,
    :matrix   => matrix

Don’t forget to add routes for the disjointed and disjointed_matrix actions. The view app/views/departures/disjointed.html.erb is simply a copy of index.html.erb with the route changed for the data call:


Now for the Departure model. You’ve seen the full query, and you saw how we broke it up to create the materialized view. We put the pieces back together in Departure#disjointed_matrix.

def self.disjointed_matrix
  sql = <<-SQL.strip_heredoc
    SELECT dest, next_origin, count(*)
    FROM aa_departures
    WHERE dest <> next_origin
      AND next_origin IS NOT NULL
    GROUP BY 1, 2
    ORDER BY 1, 2
  counts = connection.execute(sql)
  airports_matrix!(:counts => counts, :field1 => "dest", :field2 => "next_origin")

You may have noticed that we now have three parameters in the DepartureMatrix#airports_matrix! call. We need to update that method. The modified code is below with the updated lines in bold.

def airports_matrix!(counts:, field1: "origin", field2: "dest")
  h_matrix = counts.each_with_object({}) do |record, hash|
    hash[record[field1]] ||= Hash.new(0)
    hash[record[field1]][record[field2]] = Integer(record["count"])
  airports = h_matrix.keys.sort
  total    = Float(h_matrix.values.flat_map(&:values).sum)
  matrix   = Matrix.build(airports.count) do |row, column|
    origin = airports[row]
    dest   = airports[column]
    h_matrix.fetch(origin, {}).fetch(dest, 0) / total
  [airports, matrix]

The field names in the queries are different, but the matrix creation routine is the same. We can simply pass in the data’s hash key attributes, and everything still works.

You’ll need to restart the Rails app if you still had it running because you’ve added new files. Go to http://localhost:3000/departures/disjointed and you should see a chord chart that looks like Figure 6.3.

Figure 6.3

Figure 6.3 Disjointed City Pair Chord Chart

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020