I can guarantee that the data in this chart comes from the Census -- see question #2 here:
https://www.census.gov/history/www/through_the_decades/index_of_questions/1990_housing.html
Which means it tells us little about the actual density of existing areas, and is probably simply sorted by MSA or CSA instead of a set group of defined zip codes (which would account for the sudden drop from 1970-1980 when the baby boomers bought houses and suburbs rapidly expanded, and why the percentage of dense housing fell in almost every city)
So maybe you can compare relative density of metro areas?
And as a side note re: this data and other data about city density -- A weird part of LA is that it is multiple statistical areas that have merged over the years, so LA proper is dense but seamlessly meshed into suburban sprawl. If you live in LA you are not just dealing with traffic and sprawl from the very dense LA statistical area, but also from surrounding areas (like Riverside, which is also represented on that chart and whose general lack of density is more representative of the areas surrounding LA.) Every city on that chart has the same issue, some just include more of their sprawl in their core statistical area. Judging a city's worth or relative urbanity by these statistics is absolutely worthless, IMHO.