Repository: yt-project/yt

Issue:
bug set_colorbar_minorticks callback has no effect
### Bug report

**Bug summary**
title.


**Code for reproduction**

```python
import yt
ds = yt.testing.fake_amr_ds()
p = yt.SlicePlot(ds, "z", "Density")
p.set_colorbar_minorticks("Density", True)
```

**Actual outcome**
![coucou_Slice_z_density](https://user-images.githubusercontent.com/14075922/79329348-edded300-7f17-11ea-9bcd-923d9454ee1c.png)

**Expected outcome**

The colorbar minor ticks should be visible.

**Version Information**
  * Operating System: macOS
  * Python Version: 3.8.2
  * yt version: 3.6dev1
  * matplotlib version: 3.1.3


Discussion:

Resolving patch:
diff --git a/yt/visualization/plot_window.py b/yt/visualization/plot_window.py
--- a/yt/visualization/plot_window.py
+++ b/yt/visualization/plot_window.py
@@ -1052,20 +1052,32 @@ def _setup_plots(self):
             # colorbar minorticks
             if f not in self._cbar_minorticks:
                 self._cbar_minorticks[f] = True
-            if (self._cbar_minorticks[f] is True and MPL_VERSION < LooseVersion('2.0.0')
-                or self._field_transform[f] == symlog_transform):
+
+            if self._cbar_minorticks[f] is True:
+                vmin = np.float64(self.plots[f].cb.norm.vmin)
+                vmax = np.float64(self.plots[f].cb.norm.vmax)
+
                 if self._field_transform[f] == linear_transform:
                     self.plots[f].cax.minorticks_on()
-                else:
-                    vmin = np.float64( self.plots[f].cb.norm.vmin )
-                    vmax = np.float64( self.plots[f].cb.norm.vmax )
-                    if self._field_transform[f] == log_transform:
-                        mticks = self.plots[f].image.norm( get_log_minorticks(vmin, vmax) )
-                    else: # symlog_transform
-                        flinthresh = 10**np.floor( np.log10( self.plots[f].cb.norm.linthresh ) )
-                        mticks = self.plots[f].image.norm( get_symlog_minorticks(flinthresh, vmin, vmax) )
+                
+                elif self._field_transform[f] == symlog_transform:
+                    flinthresh = 10**np.floor(np.log10(self.plots[f].cb.norm.linthresh))
+                    mticks = self.plots[f].image.norm(get_symlog_minorticks(flinthresh, vmin, vmax))
                     self.plots[f].cax.yaxis.set_ticks(mticks, minor=True)
-            else:
+
+                elif self._field_transform[f] == log_transform:
+                    if MPL_VERSION >= LooseVersion('3.0.0'):
+                        self.plots[f].cax.minorticks_on()
+                        self.plots[f].cax.xaxis.set_visible(False)
+                    else:
+                        mticks = self.plots[f].image.norm(get_log_minorticks(vmin, vmax))
+                        self.plots[f].cax.yaxis.set_ticks(mticks, minor=True)
+
+                else:
+                    mylog.error("Unable to draw cbar minorticks for field {} with transform {} ".format(f, self._field_transform[f]))
+                    self._cbar_minorticks[f] = False
+
+            if not self._cbar_minorticks[f]:
                 self.plots[f].cax.minorticks_off()
 
             if draw_axes is False:


Test patch:
diff --git a/tests/tests.yaml b/tests/tests.yaml
--- a/tests/tests.yaml
+++ b/tests/tests.yaml
@@ -51,7 +51,7 @@ answer_tests:
   local_owls_002:
     - yt/frontends/owls/tests/test_outputs.py
 
-  local_pw_027:
+  local_pw_028:
     - yt/visualization/tests/test_plotwindow.py:test_attributes
     - yt/visualization/tests/test_plotwindow.py:test_attributes_wt
     - yt/visualization/tests/test_particle_plot.py:test_particle_projection_answers
@@ -71,7 +71,7 @@ answer_tests:
     - yt/analysis_modules/photon_simulator/tests/test_spectra.py
     - yt/analysis_modules/photon_simulator/tests/test_sloshing.py
 
-  local_unstructured_010:
+  local_unstructured_011:
     - yt/visualization/volume_rendering/tests/test_mesh_render.py:test_composite_mesh_render
     - yt/visualization/volume_rendering/tests/test_mesh_render.py:test_composite_mesh_render_pyembree
     - yt/visualization/volume_rendering/tests/test_mesh_render.py:test_hex20_render
@@ -128,7 +128,7 @@ answer_tests:
   local_axialpix_006:
     - yt/geometry/coordinates/tests/test_axial_pixelization.py:test_axial_pixelization
 
-  local_cylindrical_background_001:
+  local_cylindrical_background_002:
     - yt/geometry/coordinates/tests/test_cylindrical_coordinates.py:test_noise_plots
 
   local_particle_trajectory_001: