diff --git a/ggplot/geoms/geom_hline.py b/ggplot/geoms/geom_hline.py
index 51c4661d..58fa293d 100755
--- a/ggplot/geoms/geom_hline.py
+++ b/ggplot/geoms/geom_hline.py
@@ -28,7 +28,13 @@ def plot(self, ax, data, _aes):
         (data, _aes) = self._update_data(data, _aes)
         params = self._get_plot_args(data, _aes)
         variables = _aes.data
-        y = self.params.get('y')
+        y = self.params.get('yintercept')
+        # for backwards compatibility, allow setting 'y'
+        if y is None:
+            y = self.params.get('y')    
+        if y is None:
+            raise ValueError("yintercept is required for hline")
+        
         if is_iterable(y):
             for yi in y:
                 ax.axhline(yi, **params)
diff --git a/ggplot/geoms/geom_vline.py b/ggplot/geoms/geom_vline.py
index 89f5dfe1..d1629b1d 100755
--- a/ggplot/geoms/geom_vline.py
+++ b/ggplot/geoms/geom_vline.py
@@ -27,8 +27,13 @@ def plot(self, ax, data, _aes):
         (data, _aes) = self._update_data(data, _aes)
         params = self._get_plot_args(data, _aes)
         variables = _aes.data
-        x = self.params.get('x')
-
+        x = self.params.get('xintercept')
+        # for backwards compatibility, allow setting 'x'
+        if x is None:
+            x = self.params.get('x')    
+        if x is None:
+            raise ValueError("xintercept is required for vline")
+        
         if is_iterable(x):
             for xi in x:
                 ax.axvline(xi, **params)