adds test for quantile regression (#3774)

* adds test for quantile regression

* restyled

Author: kbodwin

tests/testthat/Rplot001.png

@@ -0,0 +1,53 @@
+context("geom-quantile")
+
+test_that("geom_quantile matches quantile regression", {
+  set.seed(6531)
+  x <- rnorm(10)
+  df <- tibble::tibble(
+    x = x,
+    y = x^2 + 0.5 * rnorm(10)
+  )
+
+  ps <- ggplot(df, aes(x, y)) + geom_quantile()
+
+  quants <- c(0.25, 0.5, 0.75)
+
+  pred_rq <- predict(
+    quantreg::rq(y ~ x,
+      tau = quants,
+      data = df
+    ),
+    tibble::tibble(
+      x = seq(min(x), max(x), length = 100)
+    )
+  )
+
+  pred_rq <- cbind(seq(min(x), max(x), length = 100), pred_rq)
+  colnames(pred_rq) <- c("x", paste("Q", quants * 100, sep = "_"))
+
+  ggplot_data <- tibble::as_tibble(layer_data(ps))
+
+  pred_rq_test_25 <- pred_rq[, c("x", "Q_25")]
+  colnames(pred_rq_test_25) <- c("x", "y")
+
+  expect_equal(
+    ggplot_data[ggplot_data$quantile == 0.25, c("x", "y")],
+    pred_rq_test_25
+  )
+
+  pred_rq_test_50 <- pred_rq[, c("x", "Q_50")]
+  colnames(pred_rq_test_50) <- c("x", "y")
+
+  expect_equal(
+    ggplot_data[ggplot_data$quantile == 0.5, c("x", "y")],
+    pred_rq_test_50
+  )
+
+  pred_rq_test_75 <- pred_rq[, c("x", "Q_75")]
+  colnames(pred_rq_test_75) <- c("x", "y")
+
+  expect_equal(
+    ggplot_data[ggplot_data$quantile == 0.75, c("x", "y")],
+    pred_rq_test_75
+  )
+})