r - 使用 ggmap 绘制多个 map

Question

我可以用 ggmap 绘制英国 map ，其中一个点如下:

library(ggmap)
UK_map <- get_map(location = c(-2.65, 53.7), zoom = 5, maptype = "hybrid")
UK_map <- ggmap(ggmap=UK_map, extent = "device", legend = "right")
UK_map + geom_point(data = data.frame(x = -1.81, y = 55.655), aes(x, y), size  = 5)

但是，如果我尝试使用 Winston Chang's multiplot function , 点消失。

multiplot <- function(..., plotlist=NULL, cols) {
    require(grid)

    # Make a list from the ... arguments and plotlist
    plots <- c(list(...), plotlist)

    numPlots = length(plots)

    # Make the panel
    plotCols = cols                          # Number of columns of plots
    plotRows = ceiling(numPlots/plotCols) # Number of rows needed, calculated from # of cols

    # Set up the page
    grid.newpage()
    pushViewport(viewport(layout = grid.layout(plotRows, plotCols)))
    vplayout <- function(x, y)
        viewport(layout.pos.row = x, layout.pos.col = y)

    # Make each plot, in the correct location
    for (i in 1:numPlots) {
        curRow = ceiling(i/plotCols)
        curCol = (i-1) %% plotCols + 1
        print(plots[[i]], vp = vplayout(curRow, curCol ))
    }

}

multiplot(UK_map, UK_map, cols = 2)

为什么点消失了，我怎样才能在使用 multiplot 时让点出现？

Answer 1

multiplot 函数不知道该点，因为您只将您的 UK_map 对象传递给它，而该对象不包含该点。要让它绘制点，您需要将 geom_point 调用添加到 UK_map 的赋值中，如下所示:

UK_map_with_point <- UK_map + 
  geom_point(data = data.frame(x = -1.81, y = 55.655), aes(x, y), size  = 5)

multiplot(UK_map_with_point, UK_map, cols = 2)

或者，或者，在对 multiplot 的调用中即时添加点:

multiplot(UK_map + geom_point(data = data.frame(x = -1.81, y = 55.655), 
                              aes(x, y), size  = 5), 
          UK_map + geom_point(data = data.frame(x = -2.81, y = 56.655), 
                              aes(x, y), size  = 5), cols = 2)