我用R语言ggplot画地图,但是最后的图很模糊,是以小点的形式存在,要怎么才能把小点连续起来,图片的颜色连续呢?

这边是我的代码:
ggplot() +
geom_sf(data = robin_outline, fill = "white", color = NA, linewidth = 0.5) +
geom_sf(data = world2, fill = NA, color = "grey80", linewidth = 0.25) +
geom_sf(data = grat_robin, color = "grey90", linewidth = 0.15) +
geom_tile(data = df_set_21s_robin[[i]], aes(x = long, y = lat, fill = cvalue),height = 60000,width = 60000,na.rm = T) +
geom_sf(data = robin_outline, fill = NA, color = "grey60", linewidth = 0.5) +
scale_fill_viridis_c(direction = -1, na.value = "grey80", name = name_color_21s[i]) +
coord_sf(datum = sf::st_crs("+proj=robin"))