diff --git a/seaborn/_new_core.py b/seaborn/_new_core.py
index ff58931673..8accba3ce5 100644
--- a/seaborn/_new_core.py
+++ b/seaborn/_new_core.py
@@ -1,13 +1,24 @@
+from __future__ import annotations
+from pandas import DataFrame
 
 
 class Plot:
 
     def __init__(self, data=None, **variables):
 
+        # Note that we can't assume wide-form here if variables does not contain x or y
+        # because those might get assigned in long-form fashion per layer.
+
         self.data = DataSource(data, variables)
         self.layers = []
 
-    def add(self, mark, stat=None, data=None, variables=None):
+    def add(
+        self,
+        mark: Mark,
+        stat: Stat = None,
+        data: DataFrame = None,
+        variables: dict[str, str] = None,
+    ):
 
         # TODO what if in wide-form mode, we convert to long-form
         # based on the transform that mark defines?
@@ -24,6 +35,24 @@ def plot(self):
         for layer in self.layers:
             layer.plot()
 
+    def _plot_layer(self):
+
+        # Roughly ...
+
+        # TODO where does this method come from?
+        data = self.as_numeric(self.data)
+
+        # TODO who owns the groupby logic?
+        data = self.stat(data)
+
+        # Our statistics happen on the scale we want, but then matplotlib is going
+        # to re-handle the scaling, so we need to invert before handing off
+        # Note: we don't need to convert back to strings for categories (but we could?)
+        data = self.invert_scale(data)
+
+        # Something like this?
+        self.mark.plot(data)  # do we pass ax (and/or facets?! here?)
+
     def show(self):
 
         # TODO guard this here?
@@ -42,48 +71,46 @@ def _repr_html_(self):
         pass
 
 
-class Layer:
-
-    def __init__(self, data, mark, stat):
-
-        self.data = data
-        self.mark = mark
-        self.stat = stat
-
-    def plot(self):
-
-        # Do we need this method, or can Plot just have a plot_layer method that
-        # each entry in the layers list gets passed to? (Meaning that Layer would just
-        # be a simple Dataclass or similar?
-
-        # Roughly ...
+# Do we want some sort of generator that yields a tuple of (semantics, data,
+# axes), or similar?  I guess this is basically the existing iter_data, although
+# currently the logic of getting the relevant axes lives externally (but makes
+# more sense within the generator logic). Where does this iteration happen? I
+# think we pass the generator into the Mark.plot method? Currently the plot_*
+# methods define their own grouping variables. So I guess we need to delegate to
+# them. But maybe that could be an attribute on the mark?  (Same deal for the
+# stat?)
 
-        # TODO where does this method come from?
-        data = self.as_numeric(self.data)
 
-        # TODO who owns the groupby logic?
-        data = self.stat(data)
+class DataSource:
 
-        # Our statistics happen on the scale we want, but then matplotlib is going
-        # to re-handle the scaling, so we need to invert before handing off
-        # Note: we don't need to convert back to strings for categories (but we could?)
-        data = self.invert_scale(data)
+    # How to handle wide-form data here, when the dimensional semantics are defined by
+    # the mark? (I guess? that will be most consistent with how it currently works.)
+    # I think we want to avoid too much deferred execution or else tracebacks are going
+    # to be confusing to follow...
 
-        # Something like this?
-        self.mark.plot(data)  # do we pass ax (and/or facets?! here?)
+    # With wide-form data, should we allow marks with distinct wide_form semantics?
+    # I think in most cases that will not make sense? When to check?
 
+    # I guess more generally, what to do when different variables are assigned in
+    # different calls to Plot.add()? This has to be possible (otherwise why allow it)?
+    # What does ggplot do here?
 
-class DataSource:
+    # Who owns the existing VectorPlotter.variables, VectorPlotter.var_levels, etc.?
 
     def __init__(self, data, variables):
 
         pass
 
-    def join(self, other):
+    def join(self, other: DataSource):
 
         pass
 
 
+class Stat:
+
+    pass
+
+
 class Mark:
 
     pass
@@ -95,3 +122,15 @@ def plot(self):
 
         # TODO how do we end up with ax, data, etc?
         pass
+
+
+class Layer:
+
+    # Does this need to be anything other than a simple container for these attributes?
+    # Could use a Dataclass I guess?
+
+    def __init__(self, data: DataSource, mark: Mark, stat: Stat):
+
+        self.data = data
+        self.mark = mark
+        self.stat = stat