Cherry-pick PR nschloe#623: Add mark size for legends

Author: henrypfister

src/tikzplotlib/_legend.py

@@ -63,6 +63,29 @@ def draw_legend(data, obj):
     if fill_xcolor != "white":  # white is default
         legend_style.append(f"fill={fill_xcolor}")
 
+    mark_options = []
+    handles = (
+        obj.legendHandles
+        if hasattr(obj, "legendHandles")
+        else obj.legend_handles if hasattr(obj, "legend_handles") else None
+    )
+    if handles and any(hasattr(handle, "_sizes") for handle in handles):
+        handles_with_sizes = [handle for handle in handles if hasattr(handle, "_sizes")]
+        all_sizes = set(sz for handle in handles_with_sizes for sz in handle._sizes)
+        if len(all_sizes) > 1:
+            warnings.warn(
+                f"Varying marker sizes in the legend: {all_sizes}. Ignoring all of them."
+            )
+        elif all_sizes:
+            mark_size = all_sizes.pop()
+            ff = data["float format"]
+            # setting half size because pgfplots counts the radius/half-width, and sqrt the area
+            pgf_size = 0.5 * mark_size**0.5
+            mark_options.append(f"mark size={pgf_size:{ff}}")
+
+    if mark_options:
+        legend_style.append(f"mark options={{{', '.join(mark_options)}}}")
+
     # Get the horizontal alignment
     try:
         alignment = children_alignment[0]