除非每个因子具有多个值，否则将其转换为因子并不适合绘制-它会尝试绘制箱形图样式的图。例如，对于同一因子中的10个观测，col = "red"颜色将显示为填充：

set.seed(123)
fact_example <- data.frame(factvar = as.factor(rep(LETTERS[1:3], 10)),
                           numvar = runif(30))

plot(fact_example$factvar, fact_example$numvar,
     col = "red")

由于每个因子只有一个观测值，这并不理想，因为它只是向您显示箱形图将生成的线。
您可以使用border = "red：

plot(quarterly_analysis$Quarter, 
     quarterly_analysis$AvgDefault, border="red")

或者，如果你想要更大的灵活性，你可以用数字来绘制它，并做一些小的调整以获得更多的控制（例如，可以改变pch，或者使它成为一个线图）：

# make numeric x values to plot
x_vals <- as.numeric(substr(quarterly_analysis$Quarter,1,4)) + rep(seq(0, 1, length.out = 4))

par(mfrow=c(1,3))
plot(x_vals, 
     quarterly_analysis$AvgDefault, col="red",
     pch = 7, main = "Square Symbol", axes = FALSE)
  axis(1, at = x_vals,
     labels = quarterly_analysis$Quarter)
  axis(2)

plot(x_vals, 
     quarterly_analysis$AvgDefault, col="red",
     type = "l", main = "Line graph", axes = FALSE)
  axis(1, at = x_vals,
     labels = quarterly_analysis$Quarter)
  axis(2)

plot(x_vals, 
     quarterly_analysis$AvgDefault, col="red",
     type = "b", pch = 7, main = "Both", axes = FALSE)
  axis(1, at = x_vals,
     labels = quarterly_analysis$Quarter)
  axis(2)

数据类型

set.seed(123)
quarterly_analysis <- data.frame(Quarter = as.factor(paste0(2019:2022,
                                                            rep(c(".1", ".2", ".3", ".4"), 
                                                                each = 4))),
                                 AvgDefault = runif(16))
quarterly_analysis <- quarterly_analysis[order(quarterly_analysis$Quarter),]