BUG: Fixed object type missmatch in SymLogNorm

Currently SymLogNorm accepts both linthresh and vmin, vmax with their
native dtype, meaning that the contents of one object cannot be updated
with another due to a mismatch of types.

Author: Carwyn Pelley

lib/matplotlib/colors.py

@@ -1054,7 +1054,7 @@ def __init__(self,  linthresh, linscale=1.0,
         the logarithmic range. Defaults to 1.
         """
         Normalize.__init__(self, vmin, vmax, clip)
-        self.linthresh = linthresh
+        self.linthresh = float(linthresh)
         self._linscale_adj = (linscale / (1.0 - np.e ** -1))
 
     def __call__(self, value, clip=None):
@@ -1112,7 +1112,7 @@ def _transform_vmin_vmax(self):
         Calculates vmin and vmax in the transformed system.
         """
         vmin, vmax = self.vmin, self.vmax
-        arr = np.array([vmax, vmin])
+        arr = np.array([vmax, vmin]).astype(np.float)
         self._upper, self._lower = self._transform(arr)
 
     def inverse(self, value):

lib/matplotlib/tests/test_colors.py

@@ -75,6 +75,11 @@ def test_SymLogNorm():
     _scalar_tester(norm, vals)
     _mask_tester(norm, vals)
 
+    # Ensure that specifying vmin returns the same result as above
+    norm = mcolors.SymLogNorm(3, vmin=-30, vmax=5, linscale=1.2)
+    normed_vals = norm(vals)
+    assert_array_almost_equal(normed_vals, expected)
+
 
 def _inverse_tester(norm_instance, vals):
     """