To see the code used please see the code blog.
This analysis is inspired by the book 'Python for Data Analysis' (O'Reilly) by Wes McKinney and this blog post.
There are two prominent changes in the distribution of characters for boys names in the US. The following thumb nails show the changes:
The first change started to appear in the 1960s. Before the 1960s the letters l, o and n were approximately evenly distributed. However after the 1960s the letter n becomes increasingly popular. This trend continues to the present time as the graph for 2015 shows:
The second change is more recent - starting in the late 1980s/early1990s. This time it involves the vowels a and e. Before the 1980s the letter e was more common than the letter a. That has now flipped as the graph above for 2015 shows, also the graph below for 1990:
This change in popularity for the letters a and e is not limited to male names, the same trend occurred for female names:
In the above thumbnails the two spikes on the left of each graph represent the letters a and e. Note that the letter n also became more popular for girls. Compare the two graphs below, the first is for 1880 and the second is for 1990:
However this trend for the letter n seems to be declining in more recent times, the graph below is for 2015:
While n is still ahead of l the difference is decreasing.
One further interesting comparison is the letter y which appears to be more popular in female names.