agr files with skipped set/graph numbers were not read nor written; fixes T252

src/gui_qt/graphs/graphwindow.py

@@ -530,6 +530,7 @@ class QGraphWindow(QtWidgets.QGraphicsView, Ui_GraphWindow):
             path = ''
         outfile = None
         f = FileDialog(caption='Export graphic', directory=str(path), filter=filters, mode='save')
+        f.setOption(FileDialog.DontConfirmOverwrite)
         mode = f.exec()
         if mode == QtWidgets.QDialog.Accepted:
             outfile = f.save_file()