Update the check_figures_equal testing section in CONTRIBUTING.md (#1108)

Co-authored-by: Wei Ji <[email protected]>
Co-authored-by: Meghan Jones <[email protected]>

Author: 3 people

CONTRIBUTING.md

@@ -369,35 +369,13 @@ or run tests which contain names that match a specific keyword expression:
 Writing an image-based test is only slightly more difficult than a simple test.
 The main consideration is that you must specify the "baseline" or reference
 image, and compare it with a "generated" or test image. This is handled using
-the *decorator* functions `@check_figures_equal` and
-`@pytest.mark.mpl_image_compare` whose usage are further described below.
-
-#### Using check_figures_equal
-
-This approach draws the same figure using two different methods (the reference
-method and the tested method), and checks that both of them are the same.
-It takes two `pygmt.Figure` objects ('fig_ref' and 'fig_test'), generates a png
-image, and checks for the Root Mean Square (RMS) error between the two.
-Here's an example:
-
-```python
-@check_figures_equal()
-def test_my_plotting_case():
-  "Test that my plotting function works"
-  fig_ref, fig_test = Figure(), Figure()
-  fig_ref.grdimage("@earth_relief_01d_g", projection="W120/15c", cmap="geo")
-  fig_test.grdimage(grid, projection="W120/15c", cmap="geo")
-  return fig_ref, fig_test
-```
-
-Note: This is the recommended way to test plots whenever possible, such as when
-we want to compare a reference GMT plot created from NetCDF files with one
-generated by PyGMT that passes through several layers of virtualfile machinery.
-Using this method will help save space in the git repository by not having to
-store baseline images as with the other method below.
+the *decorator* functions `@pytest.mark.mpl_image_compare` and `@check_figures_equal`
+whose usage are further described below.
 
 #### Using mpl_image_compare
 
+> **This is the preferred way to test plots whenever possible.**
+
 This method uses the [pytest-mpl](https://github.com/matplotlib/pytest-mpl)
 plug-in to test plot generating code.
 Every time the tests are run, `pytest-mpl` compares the generated plots with known
@@ -502,6 +480,24 @@ summarized as follows:
     git push
     dvc push
 
+#### Using check_figures_equal
+
+This approach draws the same figure using two different methods (the reference
+method and the tested method), and checks that both of them are the same.
+It takes two `pygmt.Figure` objects ('fig_ref' and 'fig_test'), generates a png
+image, and checks for the Root Mean Square (RMS) error between the two.
+Here's an example:
+
+```python
+@check_figures_equal()
+def test_my_plotting_case():
+  "Test that my plotting function works"
+  fig_ref, fig_test = Figure(), Figure()
+  fig_ref.grdimage("@earth_relief_01d_g", projection="W120/15c", cmap="geo")
+  fig_test.grdimage(grid, projection="W120/15c", cmap="geo")
+  return fig_ref, fig_test
+```
+
 ### Documentation
 
 #### Building the documentation