ggmap: Modified ggmap::ggmap() function
In GL-Li/ggtiger: Draw and fill in TIGER boundaries on ggmap

The extent is default to "normal" and the xlim and ylim are set in 'coord_map()' so they will not delete data outside the limits in added ggplot layers. All others are the same as ggmap::ggmap().

1 2	ggmap(ggmap, extent = "normal", base_layer, maprange = FALSE, legend = "right", padding = 0.02, darken = c(0, "black"), ...)

`ggmap`	an object of class ggmap (from function get_map)
`extent`	how much of the plot should the map take up? "normal" (default), "device", or "panel"
`base_layer`	a ggplot(aes(...), ...) call. The same as in the original ggmap().
`maprange`	logical for use with base_layer; should the map define the x and y limits? The same as in the original ggmap().
`legend`	"left", "right" (default), "bottom", "top", "bottomleft", "bottomright", "topleft", "topright", "none" (used with extent = "device"). The same as in the original ggmap().
`padding`	distance from legend to corner of the plot (used with legend, formerly b). The same as in the original ggmap().
`darken`	vector of the form c(number, color), where number is in [0, 1] and color is a character string indicating the color of the darken. 0 indicates no darkening, 1 indicates a black-out. The same as in the original ggmap().
`...`	... The same as in the original ggmap().