diff --git a/plotly/shapeannotation.py b/plotly/shapeannotation.py
index a2323ed02d4..92e1e03b504 100644
--- a/plotly/shapeannotation.py
+++ b/plotly/shapeannotation.py
@@ -4,7 +4,7 @@
 def _mean(x):
     if len(x) == 0:
         raise ValueError("x must have positive length")
-    return float(sum(x)) / len(x)
+    return float(x[0] + x[1]) / 2  # Only ever called with [x0, x1]
 
 
 def _argmin(x):
@@ -30,10 +30,10 @@ def _prepare_position(position, prepend_inside=False):
     if position is None:
         position = "top right"
     pos_str = position
-    position = set(position.split(" "))
-    if prepend_inside:
-        position = _add_inside_to_position(position)
-    return position, pos_str
+    pos_set = set(position.split(" "))
+    if prepend_inside and not ("inside" in pos_set or "outside" in pos_set):
+        pos_set.add("inside")
+    return pos_set, pos_str
 
 
 def annotation_params_for_line(shape_type, shape_args, position):
@@ -107,62 +107,44 @@ def annotation_params_for_rect(shape_type, shape_args, position):
     y0 = shape_args["y0"]
     y1 = shape_args["y1"]
 
-    position, pos_str = _prepare_position(position, prepend_inside=True)
-    if position == set(["inside", "top", "left"]):
-        return _df_anno("left", "top", min([x0, x1]), max([y0, y1]))
-    if position == set(["inside", "top", "right"]):
-        return _df_anno("right", "top", max([x0, x1]), max([y0, y1]))
-    if position == set(["inside", "top"]):
-        return _df_anno("center", "top", _mean([x0, x1]), max([y0, y1]))
-    if position == set(["inside", "bottom", "left"]):
-        return _df_anno("left", "bottom", min([x0, x1]), min([y0, y1]))
-    if position == set(["inside", "bottom", "right"]):
-        return _df_anno("right", "bottom", max([x0, x1]), min([y0, y1]))
-    if position == set(["inside", "bottom"]):
-        return _df_anno("center", "bottom", _mean([x0, x1]), min([y0, y1]))
-    if position == set(["inside", "left"]):
-        return _df_anno("left", "middle", min([x0, x1]), _mean([y0, y1]))
-    if position == set(["inside", "right"]):
-        return _df_anno("right", "middle", max([x0, x1]), _mean([y0, y1]))
-    if position == set(["inside"]):
-        # TODO: Do we want this?
-        return _df_anno("center", "middle", _mean([x0, x1]), _mean([y0, y1]))
-    if position == set(["outside", "top", "left"]):
+    pos_set, pos_str = _prepare_position(position, prepend_inside=True)
+    key = frozenset(pos_set)
+
+    # Table-driven direct hits handle most cases
+    fn = _INSIDE_OUTSIDE_TABLE.get(key, None)
+    if fn is not None:
+        return fn(x0, x1, y0, y1, shape_type)
+
+    # Special cases for "outside" positions that depend on shape_type
+    if key == frozenset(["outside", "top", "left"]):
         return _df_anno(
             "right" if shape_type == "vrect" else "left",
             "bottom" if shape_type == "hrect" else "top",
-            min([x0, x1]),
-            max([y0, y1]),
+            min(x0, x1),
+            max(y0, y1),
         )
-    if position == set(["outside", "top", "right"]):
+    if key == frozenset(["outside", "top", "right"]):
         return _df_anno(
             "left" if shape_type == "vrect" else "right",
             "bottom" if shape_type == "hrect" else "top",
-            max([x0, x1]),
-            max([y0, y1]),
+            max(x0, x1),
+            max(y0, y1),
         )
-    if position == set(["outside", "top"]):
-        return _df_anno("center", "bottom", _mean([x0, x1]), max([y0, y1]))
-    if position == set(["outside", "bottom", "left"]):
+    if key == frozenset(["outside", "bottom", "left"]):
         return _df_anno(
             "right" if shape_type == "vrect" else "left",
             "top" if shape_type == "hrect" else "bottom",
-            min([x0, x1]),
-            min([y0, y1]),
+            min(x0, x1),
+            min(y0, y1),
         )
-    if position == set(["outside", "bottom", "right"]):
+    if key == frozenset(["outside", "bottom", "right"]):
         return _df_anno(
             "left" if shape_type == "vrect" else "right",
             "top" if shape_type == "hrect" else "bottom",
-            max([x0, x1]),
-            min([y0, y1]),
+            max(x0, x1),
+            min(y0, y1),
         )
-    if position == set(["outside", "bottom"]):
-        return _df_anno("center", "top", _mean([x0, x1]), min([y0, y1]))
-    if position == set(["outside", "left"]):
-        return _df_anno("right", "middle", min([x0, x1]), _mean([y0, y1]))
-    if position == set(["outside", "right"]):
-        return _df_anno("left", "middle", max([x0, x1]), _mean([y0, y1]))
+
     raise ValueError("Invalid annotation position %s" % (pos_str,))
 
 
@@ -244,3 +226,19 @@ def split_dict_by_key_prefix(d, prefix):
         else:
             no_prefix[k] = d[k]
     return (no_prefix, with_prefix)
+
+_INSIDE_OUTSIDE_TABLE = {
+    frozenset(["inside", "top", "left"]):    lambda a,b,c,d,_: _df_anno("left",   "top",    min(a, b), max(c, d)),
+    frozenset(["inside", "top", "right"]):   lambda a,b,c,d,_: _df_anno("right",  "top",    max(a, b), max(c, d)),
+    frozenset(["inside", "top"]):            lambda a,b,c,d,_: _df_anno("center", "top",    _mean((a, b)), max(c, d)),
+    frozenset(["inside", "bottom", "left"]): lambda a,b,c,d,_: _df_anno("left",   "bottom", min(a, b), min(c, d)),
+    frozenset(["inside", "bottom", "right"]):lambda a,b,c,d,_: _df_anno("right",  "bottom", max(a, b), min(c, d)),
+    frozenset(["inside", "bottom"]):         lambda a,b,c,d,_: _df_anno("center", "bottom", _mean((a, b)), min(c, d)),
+    frozenset(["inside", "left"]):           lambda a,b,c,d,_: _df_anno("left",   "middle", min(a, b), _mean((c, d))),
+    frozenset(["inside", "right"]):          lambda a,b,c,d,_: _df_anno("right",  "middle", max(a, b), _mean((c, d))),
+    frozenset(["inside"]):                   lambda a,b,c,d,_: _df_anno("center", "middle", _mean((a, b)), _mean((c, d))),
+    frozenset(["outside", "top"]):           lambda a,b,c,d,_: _df_anno("center", "bottom", _mean((a, b)), max(c, d)),
+    frozenset(["outside", "bottom"]):        lambda a,b,c,d,_: _df_anno("center", "top",    _mean((a, b)), min(c, d)),
+    frozenset(["outside", "left"]):          lambda a,b,c,d,_: _df_anno("right",  "middle", min(a, b), _mean((c, d))),
+    frozenset(["outside", "right"]):         lambda a,b,c,d,_: _df_anno("left",   "middle", max(a, b), _mean((c, d))),
+}