Plotting: add alias --backtest-filename for --export-filename

makes it easier to discover how to use this argument

freqtrade/configuration/configuration.py

@@ -430,6 +430,15 @@ class Configuration:
         self._args_to_config(config, argname='dataformat_trades',
                              logstring='Using "{}" to store trades data.')
 
+        if self.args.get('backtestfilename'):
+            self._args_to_config(config, argname='backtestfilename',
+                             logstring='Fetching backtest results from {} ...')
+            config['backtestfilename'] = Path(config['backtestfilename'])
+        else:
+            config['backtestfilename'] = (config['user_data_dir']
+                                    / 'backtest_results')
+
+
     def _process_data_options(self, config: Dict[str, Any]) -> None:
 
         self._args_to_config(config, argname='new_pairs_days',