Scatterplot: Use opacity for contrast

Orange/widgets/visualize/owscatterplotgraph.py

@@ -1126,16 +1126,31 @@ def _get_discrete_colors(self, c_data, subset):
         c_data[np.isnan(c_data)] = len(self.palette)
         c_data = c_data.astype(int)
         colors = self.palette.qcolors_w_nan
-        pens = np.array(
-            [_make_pen(col.darker(self.DarkerValue), 1.5) for col in colors])
-        pen = pens[c_data]
-        if self.alpha_value < 255:
+        if subset is None:
+            pens = np.array(
+                [_make_pen(col.darker(self.DarkerValue), 1.5) for col in colors])
+            pen = pens[c_data]
+            if self.alpha_value < 255:
+                for col in colors:
+                    col.setAlpha(self.alpha_value)
+            brushes = np.array([QBrush(col) for col in colors])
+            brush = brushes[c_data]
+        else:
+            subset_colors = [QColor(col) for col in colors]
             for col in colors:
-                col.setAlpha(self.alpha_value)
-        brushes = np.array([QBrush(col) for col in colors])
-        brush = brushes[c_data]
-
-        if subset is not None:
+                col.setAlpha(int(224 - 0.7 * self.alpha_value))
+            for col in subset_colors:
+                col.setAlpha(128 + 0.5 * self.alpha_value)
+
+            pens, subset_pens = (
+                np.array(
+                    [_make_pen(col.darker(self.DarkerValue), 1.5)
+                     for col in cols])
+                for cols in (colors, subset_colors))
+            pen = np.where(subset, subset_pens[c_data], pens[c_data])
+
+            brushes = np.array([QBrush(col) for col in subset_colors])
+            brush = brushes[c_data]
             black = np.full(len(brush), QBrush(QColor(0, 0, 0, 0)))
             brush = np.where(subset, brush, black)
         return pen, brush