don't lose first group + fix line

Author: dcherian

xarray/plot/facetgrid.py

@@ -5,7 +5,6 @@
 import numpy as np
 
 
-from ..core.utils import peek_at
 from ..core.formatting import format_item
 from .utils import (
     _infer_xy_labels,
@@ -401,8 +400,12 @@ def map_groupby(
         func_kwargs.update({"add_colorbar": False, "add_labels": False})
 
         # Get x, y labels for the first subplot
+        grouped = self.data
+        first_group = grouped._obj.isel(
+            **{grouped._group_dim: grouped._group_indices[0]}
+        ).squeeze()
         x, y = _infer_xy_labels(
-            darray=peek_at(self.data._iter_grouped())[0],
+            darray=first_group,
             x=x,
             y=y,
             imshow=func.__name__ == "imshow",
@@ -425,9 +428,17 @@ def map_groupby_line(
     ):
         from .plot import _infer_line_data
 
+        grouped = self.data
+        first_group = grouped._obj.isel(
+            **{grouped._group_dim: grouped._group_indices[0]}
+        ).squeeze()
+        _, _, hueplt, xlabel, ylabel, huelabel = _infer_line_data(
+            darray=first_group, x=x, y=y, hue=hue
+        )
+
         for (_, subset), ax in zip(self.data, self.axes.flat):
             mappable = func(
-                subset,
+                subset.squeeze(),
                 x=x,
                 y=y,
                 ax=ax,
@@ -438,10 +449,6 @@ def map_groupby_line(
             )
             self._mappables.append(mappable)
 
-        _, _, hueplt, xlabel, ylabel, huelabel = _infer_line_data(
-            darray=peek_at(self.data._iter_grouped())[0], x=x, y=y, hue=hue
-        )
-
         self._hue_var = hueplt
         self._hue_label = huelabel
         self._finalize_grid(xlabel, ylabel)