The chart is a scattergram, with the data points representing specific election years, from 1948 to 1996. The x-axis shows the percent of House seats won by Democrats in each election year, while the y-axis shows the Democratic percent of the two-party presidential vote that year. So, for instance, 1964 was a year with a strong coattail effect. The Democratic presidential candidate (Lyndon Johnson) won a landslide victory with more than 60 percent of the vote, helping Democratic House candidates to win nearly 70 percent of the House seats. While each individual data point does not always fit the pattern, the trend line in the graph (the best fit of the data) is strongly upward, meaning that the higher the percent of the vote that the presidential candidate gets, the better the chances that the party's House members will also win; likewise, in elections where the presidential candidate fares poorly (e.g., 1952), the party's House candidates do not win as much as usual. Thus, the term "coattails" refers to a presidential candidate, dressed in a formal long coat, whose popularity or vote-getting prowse provides the long tails of a formal coat to which his party's congressional candidates can grab hold to help pull them to victory.