Fix edge case where vcenter is not properly set for diverging norms (#314)

* add unittest

* add to parse cmap

* replace unittest with becker's example

* restore init of diverging norm

* fix import

Author: cvanelteren

ultraplot/axes/plot.py

@@ -2707,6 +2707,13 @@ def _parse_cmap(
 
         # Create the continuous normalizer.
         norm = _not_none(norm, "div" if "diverging" in trues else "linear")
+        # If using a diverging norm, fair=True, and vcenter not set, default to midpoint
+        if norm in ("div", "diverging") or "diverging" in trues:
+            fair = norm_kw.get("fair", True)  # defaults to True
+            vcenter = norm_kw.get("vcenter", 0)
+            if fair and vcenter is None and vmin is not None and vmax is not None:
+                vcenter = 0.5 * (vmin + vmax)
+            norm_kw["vcenter"] = vcenter
         if isinstance(norm, mcolors.Normalize):
             norm.vmin, norm.vmax = vmin, vmax
         else:
@@ -2939,7 +2946,6 @@ def _parse_level_lim(
                     f"Incompatible arguments vmin={vmin!r}, vmax={vmax!r}, and "
                     "symmetric=True. Ignoring the latter."
                 )
-
         return vmin, vmax, kwargs
 
     def _parse_level_num(

ultraplot/tests/test_color.py

@@ -0,0 +1,37 @@
+import ultraplot as uplt, numpy as np, pytest
+
+
+@pytest.mark.mpl_image_compare
+def test_vcenter_values():
+    """
+    Test that vcenter values are correctly set in colorbars.
+    """
+    rng = np.random.default_rng(seed=10)
+    mvals = rng.normal(size=(32, 32))
+    cmap = "spectral"
+    # The middle and right plot should look the same
+    # The colors should spread out where the extremes are visible
+    fig, axs = uplt.subplots(ncols=3, share=0)
+    for i, ax in enumerate(axs):
+        specs = {}
+        if i > 0:
+            vmin = -0.2
+            vmax = 2.0
+            specs = dict(vmin=vmin, vmax=vmax)
+            if i == 2:
+                mvals = np.clip(mvals, vmin, vmax)
+        m = ax.pcolormesh(
+            mvals,
+            cmap=cmap,
+            discrete=False,
+            **specs,
+        )
+        ax.format(
+            grid=False,
+            xticklabels=[],
+            xticks=[],
+            yticklabels=[],
+            yticks=[],
+        )
+        ax.colorbar(m, loc="r", label=f"{i}")
+    return fig