Update docs

docs/developers_guide/framework.md

@@ -471,6 +471,46 @@ from an MPAS mesh file.  Optionally, you can provide the name of an MPAS field
 on cells in the mesh file that gives different weight to different cells
 (`weight_field`) in the partitioning process.
 
+(dev-visualization)=
+
+## Visualization
+
+Visualization is an optional, but desirable aspect of test cases. Often,
+visualization is an optional step of a test case but can also be included
+as part of other steps such as `initial_state` or `analysis`.
+
+While developers can write their own visualization scripts associated with
+individual test cases, the following shared visualization routines are
+provided in `polaris.viz`:
+
+{py:func}`polaris.viz.plot_horiz_field()` produces a patches-style
+visualization of x-y fields across a single vertical level at a single time
+step. The image file (png) is saved to the directory from which
+{py:func}`polaris.viz.plot_horiz_field()` is called. The function
+automatically detects whether the field specified by its variable name is
+a cell-centered variable or an edge-variable and generates the patches, the
+polygons characterized by the field values, accordingly.
+
+```{image} images/baroclinic_channel_cell_patches.png
+:align: center
+:width: 250 px
+```
+
+```{image} images/baroclinic_channel_edge_patches.png
+:align: center
+:width: 250 px
+```
+
+An example function call that uses the default vertical level (top) is:
+
+```python
+plot_horiz_field(config, ds, ds_mesh, 'normalVelocity',
+                 'final_normalVelocity.png',
+                 t_index=t_index,
+                 vmin=-max_velocity, vmax=max_velocity,
+                 cmap='cmo.balance', show_patch_edges=True)
+```
+
 (dev-validation)=
 
 ## Validation