fix y_on_right==None case (issue #190)

Author: DanielGoldfarb

src/mplfinance/_version.py

@@ -1,5 +1,5 @@
 
-version_info = (0, 12, 6, 'alpha', 0)
+version_info = (0, 12, 6, 'alpha', 1)
 
 _specifier_ = {'alpha': 'a','beta': 'b','candidate': 'rc','final': ''}
 

src/mplfinance/plotting.py

@@ -719,13 +719,11 @@ def _set_ylabels_side(ax_pri,ax_sec,primary_on_right):
         ax_pri.yaxis.tick_right()
         ax_sec.yaxis.set_label_position('left')
         ax_sec.yaxis.tick_left()
-    elif primary_on_right == False:
+    else:  # treat non-True as False, whether False, None, or anything else.
         ax_pri.yaxis.set_label_position('left')
         ax_pri.yaxis.tick_left()
         ax_sec.yaxis.set_label_position('right')
         ax_sec.yaxis.tick_right()
-    else:
-        raise ValueError('primary_on_right must be `True` or `False`')
 
 def _plot_mav(ax,config,xdates,prices,apmav=None,apwidth=None):
     style = config['style']