A striking yet somewhat confusing way to visualize this data. With the X axis being the average year of all letters sent by a person, interpreting the distance between corespondents is somewhat challenging. It's hard to know how a line from someone averaging around 1575 to someone around 1530 should be interpreted, since their letters aren't exactly time traveling. The difference could be explained by relative ages, yet I reckon there are other interesting factors to consider (corresponding with one set of people early in life, changing to another with age possibly). The timeline you get by clicking on "Explore by time" provides some more data, yet it isn't quite clear 100% there either.