This tool is used for visualizing the performance of each NFL team and quarterback to play in the league from the start of the Super Bowl Era through the 2020 season, using data from FiveThirtyEight. I invite you to read more about FiveThirtyEight's project from which this data was sourced, The Complete History of the NFL.
Both The Complete History of the NFL and this application use Elo ratings as a quantitative measure of skill for each team and quarterback. Elo ratings are a rating scheme used to qualitatively evaluate a team or player within a set pool of teams or players, such as the teams in the NFL within a single season. Elo ratings were originally developed to rank chess players relative to each other, and the history and development of this ranking system can be found at the Elo Ranking Wikipedia page. How FiveThirtyEight calculates these ratings at the beginning of each season and adjusts them as the season progresses can be found at this excellent page explaining their methodology and formulae.
Elo ratings are set at the beginning of each season using a combination of the results from the prior season and an adjustment for an assumed reversion to the mean (the mean Elo rating for an NFL tam is 1505). The ratings are then adjusted using a formula after every game based on the winner and loser of the game. Losing teams "give" points to the winning team, their score going down while the winning team's rises by the same margin. The amount of Elo points transferred in this way is determined by a formula, as opposed to the highly subjective Power Rankings many media analysts favor. The Elo ratings for each team can also be used before a game to determine the expected percentage chance that the favored team has of winning.
FiveThirtyEight produces Elo ratings every year for both NFL teams and quarterbacks, and those ratings are the subject of this visualization tool. FiveThirtyEight also calculates a QB-adjusted team Elo rating, which corrects for changes due to injury or outperformance at the game's most important position. This adjusts the team Elo rating in situations such as a high-profile starter becoming injured and being replaced by an inexperienced backup - the entire set of expectations for the team changes, and the QB-adjustment reflects that.
The tabs to the right allow a user to visualize the Elo rating trace of an NFL team or QB throughout the balance of a particular season or their lifetime in the league. The main plots in each tab allow the user to zoom in or out, or pan across the plot to get a more precise look at a subset of the data if they desire. The plots also have rollover interactivity, showing pop-up stats if the user mouses over a particular game. Enjoy!
Note: Teams are listed as their present-day incarnation. Therefore, all Houston Oilers records are listed under the Tennessee Titans team name, all St. Louis Cardinals records are listed under the Arizona Cardinals name, and so on. In addition, team names reflect the most up-to-date team naming available in the R Programming Language package teamcolors. Therefore, the Washington Football Team is listed under the name Washington Redskins, and the Las Vegas Raiders are listed under the team name Oakland Raiders. I have contacted the developer of this package about updating the team database and am awaiting a response.