made bokeh backend configurable thru scheme

Author: verybadsoldier

backtrader_plotting/bokeh/figure.py

@@ -46,7 +46,7 @@ def _init_figure(self):
         f = figure(tools=Figure._tools,
                    x_axis_type='linear',
                    aspect_ratio=self._scheme.plotaspectratio,
-                   output_backend="webgl",
+                   output_backend=self._scheme.output_backend,
                    )
 
         f.y_range.range_padding = self._scheme.y_range_padding

backtrader_plotting/schemes/btscheme.py

@@ -187,6 +187,9 @@ def __init__(self):
         # include strategy source code in the meta data
         self.strategysrc = False
 
+        # bokeh output backend
+        self.output_backend = 'webgl'
+
     def color(self, idx):
         colidx = tab10_index[idx % len(tab10_index)]
         return self.lcolors[colidx]