My houses_wide data frame has one column for Month (I’m just looking at December for each year starting in 2007) and columns for each city. Classes ‘data.table’ and 'data.frame': 14 obs. of ...

Side-by-side ggplot2 graphs with the patchwork package. It’s incredibly easy to place multiple graphs with patchwork. For more on how to customize layouts, head to the patchwork website.