Here is a chart of the number of foreign songs to hit the Year-End U.S. Billboard Hot 100 every year. Note that songs that charted for multiple years appear for each year they charted in.

Also note that these are expressed as a percent, which doesn't affect much except for in 1958 where Billboard's chart only had 50 songs.

Here is a line graph that shows the number of hit songs each Year-End U.S. Billboard Hot 100 from a few of the most notable non-American countries. If a song is made by 2 non-American acts from different countries, it is listed as one song for both countries. However, if both artists were from the same country, it counts for just one song. Features are not included. Some of the countries of origin for the acts are based on my discretion.