diff --git a/gridlock/read.py b/gridlock/read.py
index 7b4de1e..503e996 100644
--- a/gridlock/read.py
+++ b/gridlock/read.py
@@ -147,6 +147,7 @@ class GridReadMixin(GridPosMixin):
             cell_data: NDArray,
             plane: PlaneProtocol | PlaneDict,
             which_shifts: int = 0,
+            sample_period: int = 1,
             finalize: bool = True,
             contour_args: dict[str, Any] | None = None,
             ax: 'matplotlib.axes.Axes | None' = None,
@@ -207,7 +208,7 @@ class GridReadMixin(GridPosMixin):
             fig, ax = pyplot.subplots()
         else:
             fig = ax.figure
-        xc, yc = (self.shifted_exyz(which_shifts)[a] for a in surface)
+        xc, yc = (self.shifted_xyz(which_shifts)[a] for a in surface)
         xcmesh, ycmesh = numpy.meshgrid(xc, yc, indexing='ij')
 
         mappable = ax.contour(xcmesh, ycmesh, grid_slice, levels=levels, **contour_args)