创建Choropleth贴图时如何避免空白区域

Question

我正在尝试使用tm_shape包中的tmap来绘制地图，但是即使我的数据不包含缺失值，我的地图也有空白区域。

代码如下：

tm_shape(london_map)+
    tm_fill(c("ni","di","diff3","ti"), 
           style=c("jenks","pretty"),
           palette=list("Purples","Blues","YlGn","OrRd"),
           auto.palette.mapping=FALSE,
           title=c("Improved Trips","Improved Time Weighted by Trips","One-mode Population Change","Total Improved Time"))