I have two dataframes (fire2022_12 and fire2021_12) with the same three columns (variable, x, y). I have pasted a snippet from the top of these dataframes below. I can plot these two dataframes in the same plot just fine, by doing this:
ggplot() + geom_line(data=fire2022_12, aes(x, y)) + geom_line(data=fire2021_12, aes(x, y))