How to use geom line and categorical data from 6 different lists(csvs)-CodePudding

I am starting with 6 different lists/csvs that each contain one charater column. This column shows the Home Owner Loan Corporation (HOLC) neighborhood grades of census block groups. So the columns look something like shown below. I am new to using R studio and I am wondering if the first step would be to combine the lists. Another option could be to add a

If you want all the lines on the same panel, just get rid of that final facet_grid line. It looks messy without this at present because your numbers are so small.

Data in reproducible format, taken from question

Houston_2020_sub <- data.frame(houston_grade2020 = c(NA, NA, 'B', 'A', 
                                                     NA, 'C', 'D'))

Minneapolis_2020_sub <- data.frame(minneapolis_grade2020 = c('A', NA, NA, "B", 
                                                             "C", "C", "D"))

Houston_1990_sub <- data.frame(houston_grade1990 = c('B', 'B', 'B', 'A', 'A', 
                                                     'C', 'D'))

Minneapolis_1990_sub <- data.frame(minneapolis_grade1990 = c('B', 'A', NA, 'A',
                                                             NA, NA, 'D'))