```r
urb_1970_2030 = urban_agglomerations %>% 
  filter(year %in% c(1970, 1990, 2010, 2030))
tm_shape(world) + tm_polygons() + 
  tm_shape(urb_1970_2030) + tm_symbols(col = "black", border.col = "white",
                                       size = "population_millions") +
  tm_facets(by = "year", nrow = 2, free.coords = FALSE)
```

<div class="figure" style="text-align: center">
<img src="figures/urban-facet-1.png" alt="Faceted map showing the top 30 largest urban agglomerations from 1970 to 2030 based on population projects by the United Nations." width="576" />
<p class="caption">(\#fig:urban-facet)Faceted map showing the top 30 largest urban agglomerations from 1970 to 2030 based on population projects by the United Nations.</p>
</div>