Inherit errorbar alpha from scatter points in regplot

Closes #2540 by adding a test.

Author: mwaskom

doc/releases/v0.12.0.txt

@@ -53,6 +53,8 @@ Other updates
 
 - |Enhancement| When using :func:`pairplot` with `corner=True` and `diag_kind=None`, the top left y axis label is no longer hidden (:pr:2850`).
 
+- |Enhancement| Error bars in :func:`regplot` now inherit the alpha value of the points they correspond to (:pr:`2540`).
+
 - |Fix| Fixed a regression in 0.11.2 that caused some functions to stall indefinitely or raise when the input data had a duplicate index (:pr:`2776`).
 
 - |Fix| Fixed a bug in :func:`histplot` and :func:`kdeplot` where weights were not factored into the normalization (:pr:`2812`).

tests/test_regression.py

@@ -522,6 +522,14 @@ def test_regplot_scatter_kws_alpha(self):
                         scatter_kws={'color': color})
         assert ax.collections[0]._alpha == 0.8
 
+        f, ax = plt.subplots()
+        alpha = .3
+        ax = lm.regplot(x="x", y="y", data=self.df,
+                        x_bins=5, fit_reg=False,
+                        scatter_kws={"alpha": alpha})
+        for line in ax.lines:
+            assert line.get_alpha() == alpha
+
     def test_regplot_binned(self):
 
         ax = lm.regplot(x="x", y="y", data=self.df, x_bins=5)