Feature: Respect savefig rcParams in Download button

Fixes #138, #234, #339

The Download button now respects ALL matplotlib savefig.* rcParams
instead of always saving as PNG with hardcoded settings.

Backend changes:
- Add _send_save_buffer() method that calls figure.savefig() without
  hardcoded parameters (respects all rcParams)
- Add 'save_figure' message handler to intercept Download button clicks
- Send buffer + format metadata to frontend via ipywidgets comm

Frontend changes:
- Update handle_save() to accept buffers from backend
- Support multiple formats with correct MIME types:
  PNG, PDF, SVG, EPS, JPEG, TIFF, PS, TIF
- Set correct file extensions based on format
- Maintain backward compatibility with canvas.toDataURL() fallback

Respects these rcParams:
- savefig.format (png, pdf, svg, jpg, eps, etc.)
- savefig.dpi (resolution)
- savefig.transparent (transparent backgrounds)
- savefig.facecolor (custom background colors)
- All other savefig.* parameters

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: ianhi

ipympl/backend_nbagg.py

@@ -277,6 +277,9 @@ def _handle_message(self, object, content, buffers):
             _, _, w, h = self.figure.bbox.bounds
             self.manager.resize(w, h)
 
+        elif content['type'] == 'save_figure':
+            self._send_save_buffer()
+
         elif content['type'] == 'set_dpi_ratio':
             Canvas.current_dpi_ratio = content['dpi_ratio']
             self.manager.handle_json(content)
@@ -327,6 +330,29 @@ def send_binary(self, data):
         # Actually send the data
         self.send({'data': '{"type": "binary"}'}, buffers=[data])
 
+    def _send_save_buffer(self):
+        """Generate figure buffer respecting savefig rcParams and send to frontend."""
+        from matplotlib import rcParams
+
+        buf = io.BytesIO()
+
+        # Call savefig WITHOUT any parameters - fully respects all rcParams
+        self.figure.savefig(buf)
+
+        # Detect the format that was actually used
+        # Priority: explicitly set format, or rcParams, or default 'png'
+        fmt = rcParams.get('savefig.format', 'png')
+
+        # Get the buffer data
+        data = buf.getvalue()
+
+        # Send to frontend with format metadata
+        msg_data = {
+            "type": "save",
+            "format": fmt
+        }
+        self.send({'data': json.dumps(msg_data)}, buffers=[data])
+
     def new_timer(self, *args, **kwargs):
         return TimerTornado(*args, **kwargs)
 

src/mpl_widget.ts

@@ -148,13 +148,58 @@ export class MPLCanvasModel extends DOMWidgetModel {
         }
     }
 
-    handle_save() {
-        const save = document.createElement('a');
-        save.href = this.offscreen_canvas.toDataURL();
-        save.download = this.get('_figure_label') + '.png';
-        document.body.appendChild(save);
-        save.click();
-        document.body.removeChild(save);
+    handle_save(msg?: any, buffers?: (ArrayBuffer | ArrayBufferView)[]) {
+        // If called with buffers (new path), use the backend-generated buffer
+        if (buffers && buffers.length > 0) {
+            const url_creator = window.URL || window.webkitURL;
+
+            // Get format from message (already parsed by on_comm_message)
+            const format = msg.format || 'png';
+
+            // Convert buffer to Uint8Array
+            const buffer = new Uint8Array(
+                ArrayBuffer.isView(buffers[0]) ? buffers[0].buffer : buffers[0]
+            );
+
+            // Map format to MIME type
+            const mimeTypes: { [key: string]: string } = {
+                'png': 'image/png',
+                'jpg': 'image/jpeg',
+                'jpeg': 'image/jpeg',
+                'pdf': 'application/pdf',
+                'svg': 'image/svg+xml',
+                'eps': 'application/postscript',
+                'ps': 'application/postscript',
+                'tif': 'image/tiff',
+                'tiff': 'image/tiff'
+            };
+
+            const mimeType = mimeTypes[format] || 'application/octet-stream';
+
+            // Create blob with correct MIME type
+            const blob = new Blob([buffer], { type: mimeType });
+            const blob_url = url_creator.createObjectURL(blob);
+
+            // Create download link with correct extension
+            const save = document.createElement('a');
+            save.href = blob_url;
+            save.download = this.get('_figure_label') + '.' + format;
+            document.body.appendChild(save);
+            save.click();
+            document.body.removeChild(save);
+
+            // Clean up the blob URL
+            url_creator.revokeObjectURL(blob_url);
+        } else {
+            // Fallback to old behavior (use canvas toDataURL)
+            // This maintains backward compatibility
+            const save = document.createElement('a');
+            save.href = this.offscreen_canvas.toDataURL();
+            save.download = this.get('_figure_label') + '.png';
+            document.body.appendChild(save);
+            save.click();
+            document.body.removeChild(save);
+        }
     }
 
     handle_resize(msg: { [index: string]: any }) {

src/toolbar_widget.ts

@@ -144,6 +144,14 @@ export class ToolbarView extends DOMWidgetView {
 
     toolbar_button_onclick(name: string) {
         return (_event: Event): void => {
+            // Special case for save_figure - send custom message for rcParams handling
+            if (name === 'save_figure') {
+                this.send({
+                    type: 'save_figure',
+                });
+                return;
+            }
+
             // Special case for pan and zoom as they are toggle buttons
             if (name === 'pan' || name === 'zoom') {
                 if (this.model.get('_current_action') === name) {