Added span parameter for datashade operation

holoviews/operation/datashader.py

@@ -337,6 +337,11 @@ class shade(Operation):
         and any valid transfer function that accepts data, mask, nbins
         arguments.""")
 
+    span = param.NumericTuple(default=None, length=2, doc="""
+        Min and max data values to use for colormap interpolation, when
+        wishing to override autoranging.
+        """)
+
     link_inputs = param.Boolean(default=True, doc="""
          By default, the link_inputs parameter is set to True so that
          when applying shade, backends that support linked streams
@@ -409,6 +414,8 @@ def _process(self, element, key=None):
             shade_opts['cmap'] = map(self.rgb2hex, colors)
         else:
             shade_opts['cmap'] = self.p.cmap
+        if self.p.span:
+            shade_opts['span'] = self.p.span
 
         with warnings.catch_warnings():
             warnings.filterwarnings('ignore', r'invalid value encountered in true_divide')