从下面生成的地图非常小（几乎看不到）。有办法“放大”吗？
library(tmap)
library(maptools)
library(tigris)
a=read_shape("sub_div.shp", as.sf=TRUE, stringsAsFactors=FALSE)
city.list=c("Mississauga", "Toronto", "Brampton", "Etobicoke","Markham", "Richmond Hill", "Oshawa", "Pickering",
"Whitby", "Ajax", "Caledon", "Oakville", "Burlington", "Halton Hills", "Milton", "King", "Vaughan",
"East Gwillimbury", "Whitchurch-Stouffville", "Brock", "Uxbridge", "Scugog", "Clarington", "Aurora",
"Newmarket", "Georgina")
gta= a %>% filter(CSDNAME %in% city.list)
tm_shape(gta)+tm_polygons()