Option for gradient coloring and alpha in edges (#36)

* Include fransua's gradient coloring and alpha edges code

pysankey/sankey/__init__.py

@@ -22,6 +22,7 @@
 from collections import defaultdict
 from typing import Any, Dict, List, Optional, Set, Tuple, Union
 
+import matplotlib as mpl
 import matplotlib.pyplot as plt
 import numpy as np
 import pandas as pd
@@ -72,6 +73,8 @@ def sankey(
     closePlot: bool = False,
     figSize: Optional[Tuple[int, int]] = None,
     ax: Optional[Any] = None,
+    color_gradient: bool = False,
+    alphaDict: Optional[Dict[Union[str, Tuple[str, str]], float]] = None,
 ) -> Any:
     """
     Make Sankey Diagram showing flow from left-->right
@@ -128,6 +131,20 @@ def sankey(
     rightWidths, topEdge = _get_positions_and_total_widths(
         data_frame, rightLabels, "right"
     )
+    # If no alphaDict given, make one
+    if alphaDict is None:
+        alphaDict = {}
+        for _, label in enumerate(all_labels):
+            alphaDict[label] = 0.65
+    else:
+        missing = [label for label in all_labels if label not in alphaDict.keys()]
+        if missing:
+            msg = (
+                "The alphaDict parameter is missing values for the following labels : "
+            )
+            msg += ", ".join(missing)
+            raise ValueError(msg)
+    LOGGER.debug("The alphadict value are : %s", alphaDict)
     # Total vertical extent of diagram
     xMax = topEdge / aspect
     draw_vertical_bars(
@@ -152,6 +169,8 @@ def sankey(
         rightLabels,
         rightWidths,
         xMax,
+        alphaDict,
+        color_gradient,
     )
     if figSize is not None:
         plt.gcf().set_size_inches(figSize)
@@ -353,7 +372,10 @@ def _create_dataframe(
 
 def plot_strips(
     ax: Any,
-    colorDict: Union[Dict[str, Tuple[float, float, float]], Dict[str, str]],
+    colorDict: Union[
+        Dict[Union[str, Tuple[str, str]], Tuple[float, float, float]],
+        Dict[Union[str, Tuple[str, str]], str],
+    ],
     dataFrame: DataFrame,
     leftLabels: ndarray,
     leftWidths: Dict,
@@ -363,6 +385,8 @@ def plot_strips(
     rightLabels: ndarray,
     rightWidths: Dict,
     xMax: float64,
+    alphaDict: Dict[Union[str, Tuple[str, str]], float],
+    color_gradient: bool = False,
 ) -> None:
     # Plot strips
     for leftLabel in leftLabels:
@@ -398,13 +422,58 @@ def plot_strips(
                 # right place
                 leftWidths[leftLabel]["bottom"] += ns_l[leftLabel][rightLabel]
                 rightWidths[rightLabel]["bottom"] += ns_r[leftLabel][rightLabel]
-                ax.fill_between(
-                    np.linspace(0, xMax, len(ys_d)),
-                    ys_d,
-                    ys_u,
-                    alpha=0.65,
-                    color=colorDict[label_color],
-                )
+
+                if (leftLabel, rightLabel) in alphaDict:
+                    alpha = alphaDict[leftLabel, rightLabel]
+                else:
+                    alpha = alphaDict[label_color]
+                if color_gradient:
+                    if (leftLabel, rightLabel) in colorDict:
+                        cleft = cright = colorDict[leftLabel, rightLabel]
+                    else:
+                        cleft = colorDict[leftLabel]
+                        cright = colorDict[rightLabel]
+
+                    x = list(np.linspace(0, xMax, len(ys_d)))
+                    (poly,) = ax.fill(
+                        x + x[::-1] + [x[0]],
+                        list(ys_d) + list(ys_u)[::-1] + [ys_d[0]],
+                        facecolor="none",
+                    )
+
+                    # get the extent of the axes
+                    xmin, xmax = ax.get_xlim()
+                    ymin, ymax = ax.get_ylim()
+
+                    # create a dummy image
+                    img_data = np.arange(xmin, xmax, (xmax - xmin) / 100.0)
+                    img_data = img_data.reshape(img_data.size, 1).T
+
+                    # plot and clip the image
+                    im = ax.imshow(
+                        img_data,
+                        aspect="auto",
+                        origin="lower",
+                        cmap=mpl.colors.LinearSegmentedColormap.from_list(
+                            "custom", [cleft, cright]
+                        ),
+                        alpha=alpha,
+                        extent=[xmin, xmax, ymin, ymax],
+                    )
+
+                    im.set_clip_path(poly)
+                else:
+                    if (leftLabel, rightLabel) in colorDict:
+                        color = colorDict[leftLabel, rightLabel]
+                    else:
+                        color = colorDict[label_color]
+                    ax.fill_between(
+                        np.linspace(0, xMax, len(ys_d)),
+                        ys_d,
+                        ys_u,
+                        alpha=alpha,
+                        color=color,
+                    )
     ax.axis("off")