geom_path: Each group consists of only one observation. Do you need to adjust the group aesthetic?

plot5 <- ggplot(df, aes(year, pollution)) +
           geom_point() +
           geom_line() +
           labs(x = "Year", y = "Particulate matter emissions (tons)", title = "Motor vehicle emissions in Baltimore")

得到错误：

geom_path: Each group consists of only one observation. Do you need to adjust the group aesthetic?

您只需将group = 1添加到ggplot或geom_line aes（）中。

对于折线图，必须对数据点进行分组，以便知道要连接的点。在这种情况下，它很简单 - 所有点都应该连接，所以group = 1。当使用更多变量并绘制多行时，行的分组通常由变量完成。

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