diff --git a/chaco/plots/horizon_plot.py b/chaco/plots/horizon_plot.py
index ba3e37679..dd6ff0850 100644
--- a/chaco/plots/horizon_plot.py
+++ b/chaco/plots/horizon_plot.py
@@ -29,6 +29,8 @@ def map_screen(self, data_array):
             else:
                 return array([self.low_pos])
         else:
+            if not isinstance(data_array, ndarray):
+                data_array = array(data_array, ndmin=1)
             # Scale the data by the number of bands
             return (
                 data_array * self.bands - self.range.low
diff --git a/chaco/polar_mapper.py b/chaco/polar_mapper.py
index cefb76393..5142eac2a 100644
--- a/chaco/polar_mapper.py
+++ b/chaco/polar_mapper.py
@@ -62,6 +62,8 @@ def map_screen(self, data_array):
             else:
                 return array([self.low_pos])
         else:
+            if not isinstance(data_array, ndarray):
+                data_array = array(data_array, ndmin=1)
             return (data_array - self.range.low) * self._scale + self.low_pos
 
     def map_data(self, screen_val):