Fix svg colorbars

plotnine/ggplot.py

@@ -129,7 +129,7 @@ def __init__(
         self.watermarks: list[watermark] = []
 
         # build artefacts
-        self._build_objs = NS()
+        self._build_objs = NS(meta={})
 
     def __str__(self) -> str:
         """
@@ -595,6 +595,9 @@ def save_helper(
         This method has the same arguments as [](`~plotnine.ggplot.save`).
         Use it to get access to the figure that will be saved.
         """
+        if format is None and isinstance(filename, (str, Path)):
+            format = str(filename).split(".")[-1]
+
         fig_kwargs: Dict[str, Any] = {"format": format, **kwargs}
 
         if limitsize is None:
@@ -644,6 +647,7 @@ def save_helper(
         if dpi is not None:
             self.theme = self.theme + theme(dpi=dpi)
 
+        self._build_objs.meta["figure_format"] = format
         figure = self.draw(show=False)
         return mpl_save_view(figure, fig_kwargs)
 

plotnine/guides/guide.py

@@ -156,6 +156,12 @@ def train(
         Returns guide if training is successful
         """
 
+    def merge(self, other: Self) -> Self:
+        """
+        Merge with another guide
+        """
+        return self
+
     def draw(self) -> PackerBase:
         """
         Draw guide

plotnine/guides/guide_colorbar.py

@@ -28,6 +28,7 @@
     from matplotlib.text import Text
 
     from plotnine import theme
+    from plotnine.guides import guides
     from plotnine.scales.scale import scale
     from plotnine.typing import Side
 
@@ -50,7 +51,12 @@ class guide_colorbar(guide):
     """
 
     display: Literal["gradient", "rectangles", "raster"] = "gradient"
-    """How to render the colorbar."""
+    """
+    How to render the colorbar
+
+    SVG figures will always use "rectangles" to create gradients. This has
+    better support across applications that render svg images.
+    """
 
     alpha: Optional[float] = None
     """
@@ -76,6 +82,12 @@ def __post_init__(self):
         if self.nbin is None:
             self.nbin = 300  # if self.display == "gradient" else 300
 
+    def setup(self, guides: guides):
+        super().setup(guides)
+        # See: add_segmented_colorbar
+        if guides.plot._build_objs.meta.get("figure_format") == "svg":
+            self.display = "rectangles"
+
     def train(self, scale: scale, aesthetic=None):
         self.nbin = cast("int", self.nbin)
         self.title = cast("str", self.title)
@@ -123,12 +135,6 @@ def train(self, scale: scale, aesthetic=None):
         self.hash = hashlib.sha256(info.encode("utf-8")).hexdigest()
         return self
 
-    def merge(self, other):
-        """
-        Simply discards the other guide
-        """
-        return self
-
     def create_geoms(self):
         """
         Return self if colorbar will be drawn and None if not
@@ -179,6 +185,7 @@ def draw(self):
         nbars = len(self.bar)
         elements = self.elements
         raster = self.display == "raster"
+        alpha = self.alpha
 
         colors = self.bar["color"].tolist()
         labels = self.key["label"].tolist()
@@ -227,9 +234,9 @@ def draw(self):
 
         # colorbar
         if self.display == "rectangles":
-            add_segmented_colorbar(auxbox, colors, elements)
+            add_segmented_colorbar(auxbox, colors, alpha, elements)
         else:
-            add_gradient_colorbar(auxbox, colors, elements, raster)
+            add_gradient_colorbar(auxbox, colors, alpha, elements, raster)
 
         # ticks
         visible = slice(
@@ -272,6 +279,7 @@ def draw(self):
 def add_gradient_colorbar(
     auxbox: AuxTransformBox,
     colors: Sequence[str],
+    alpha: float | None,
     elements: GuideElementsColorbar,
     raster: bool = False,
 ):
@@ -327,6 +335,7 @@ def add_gradient_colorbar(
         shading="gouraud",
         cmap=cmap,
         array=Z.ravel(),
+        alpha=alpha,
         rasterized=raster,
     )
     auxbox.add_artist(coll)
@@ -335,6 +344,7 @@ def add_gradient_colorbar(
 def add_segmented_colorbar(
     auxbox: AuxTransformBox,
     colors: Sequence[str],
+    alpha: float | None,
     elements: GuideElementsColorbar,
 ):
     """
@@ -343,6 +353,21 @@ def add_segmented_colorbar(
     from matplotlib.collections import PolyCollection
 
     nbreak = len(colors)
+    # Problem:
+    # 1. Webbrowsers do not properly render SVG with QuadMesh
+    #    colorbars. Also when the QuadMesh is "rasterized",
+    #    the colorbar is misplaced within the SVG (and pdfs!).
+    #    So SVGs cannot use `add_gradient_colobar` at all.
+    # 2. Webbrowsers do not properly render SVG with PolyCollection
+    #    colorbars when the adjacent rectangles that make up the
+    #    colorbar touch each other precisely. The "bars" appear to
+    #    be separated by lines.
+    #
+    # For a wayout, we overlap the bars. Overlapping creates artefacts
+    # when alpha < 1, but having a gradient + alpha is rare. And, we can
+    # minimise apparent artefacts by using a large overlap_factor.
+    # A value of 2 gives the best results in the rare case should alpha < 1.
+    overlap_factor = 2
     if elements.is_vertical:
         colorbar_height = elements.key_height
         colorbar_width = elements.key_width
@@ -353,6 +378,8 @@ def add_segmented_colorbar(
         for i in range(nbreak):
             y1 = i * linewidth
             y2 = y1 + linewidth
+            if i > 1:
+                y1 -= linewidth * overlap_factor
             verts.append(((x1, y1), (x1, y2), (x2, y2), (x2, y1)))
     else:
         colorbar_width = elements.key_height
@@ -364,12 +391,15 @@ def add_segmented_colorbar(
         for i in range(nbreak):
             x1 = i * linewidth
             x2 = x1 + linewidth
+            if i > 1:
+                x1 -= linewidth * overlap_factor
             verts.append(((x1, y1), (x1, y2), (x2, y2), (x2, y1)))
 
     coll = PolyCollection(
         verts,
         facecolors=colors,
         linewidth=0,
+        alpha=alpha,
         antialiased=False,
     )
     auxbox.add_artist(coll)