You want to make a heat map.

Use geom_tile() or geom_raster() and map a continuous variable to fill. We’ll use the presidents data set, which is a time series object rather than a data frame:

presidents

     Qtr1 Qtr2 Qtr3 Qtr4
1945   NA   87   82   75
1946   63   50   43   32
 ...
1973   68   44   40   27
1974   28   25   24   24

str(presidents)

Time-Series [1:120] from 1945 to 1975: NA 87 82 75 63 50 43 32 35 60 ...

We’ll first convert it to a format that is usable by ggplot()—a data frame with columns that are numeric:

pres_rating <- data.frame(
    rating  = as.numeric(presidents),
    year    = as.numeric(floor(time(presidents))),
    quarter = as.numeric(cycle(presidents))
)

pres_rating

 rating year quarter
     NA 1945       1
     87 1945       2
     82 1945       3
 ...
     25 1974       2
     24 1974       3
     24 1974       4