R错误：提供给连续刻度的离散值

Question

我对这种R语言很陌生。经过一番努力，我无法解决下面提供的代码问题。请告诉我我做错了什么

library(ggmap)
library(ggplot2)

map <- get_map(location = 'India', zoom = 4) 

airports <- read.csv("C:/Users/Sumedha/Downloads/in-airports.csv", header=T)

## Listing out first few rows
head(airports)
# 
# ## I'll be using latitude_deg and longitude_deg columns to plot the points on the map
# ## geom_point() is a ggplot2 function and it plots the points on the top of the map

variablePoints <- ggmap(map) + geom_point(aes(x = longitude_deg, y = latitude_deg, size = elevation_ft), data = airports, alpha = .5)

## Modify the legend name
updatedMap <- variablePoints + scale_size_area(name = "Elevation Level (in feet)")

updatedMap

但是我收到了这个错误..

  

错误：提供给连续刻度的离散值