add docstring and news

Author: yucongalicechen

news/xtype.rst

@@ -0,0 +1,23 @@
+**Added:**
+
+* functionality to return the 2D array based on the specified xtype
+
+**Changed:**
+
+* <news item>
+
+**Deprecated:**
+
+* <news item>
+
+**Removed:**
+
+* <news item>
+
+**Fixed:**
+
+* <news item>
+
+**Security:**
+
+* <news item>

src/diffpy/utils/diffraction_objects.py

@@ -376,16 +376,17 @@ def scale_to(self, target_diff_object, xtype=None, xvalue=None):
         return scaled
 
     def on_xtype(self, xtype):
-        """
+        f"""
         return a 2D np array with x in the first column and y in the second for x of type type
 
         Parameters
         ----------
-        xtype
+        xtype str
+            the type of quantity for the independent variable from {*XQUANTITIES, }
 
         Returns
         -------
-
+        a 2D np array with x and y data
         """
         if xtype.lower() in ANGLEQUANTITIES:
             return self.on_tth()