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我完全是新手R，我想在两个国家之间画一条线（可能是加权的，例如，通过旅行次数）。目前，我使用每个首都的经度和纬度来画一条线，但我想使用包 ggmap 来做。我环顾四周，但到目前为止没有找到任何解决方案。我将不胜感激。

    require(ggmap)
    require (rworldmap)

    all_content = readLines("ext_lt_intratrd_1_Data.csv")
    skip_second = all_content[-2]
    dat = read.csv(textConnection(skip_second), header = TRUE, stringsAsFactors =F)
    dat[5,2]<- c("Germany") # using a data where the first line is
                             header, but second line must be skipped as it is EU 27
                             and not a single country
    europe <- read.csv("eulonglat.csv", header = TRUE) # using world capitals to 
    generate points
    myfulldata <- merge(dat, europe)
    map <- get_map(location = 'Europe', zoom = 4)
    mapPoints <- ggmap(map) + geom_point(aes(x = UNc_longitude, y = UNc_latitude, size 
    = log(myfulldata$Value)), data = myfulldata, col = "red", alpha= 0.5) # this can 
    be plotted
    # i would continue with drawing line and i searched for references
    # i found arrows(42.66,23.34,50.82,4.47) - which did not work 
    # i tried to look for a reference work more, but could not find
    # instead i found it using with the package rworldmap the following
   lines(c(4.47, 23.32), c(50.82, 42.66))
    # this does not work on ggmap
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