BUG: Fix crosshair linked across subplots

* https://docs.bokeh.org/en/latest/docs/examples/interaction/linking/linked_crosshair.html

Fixes #1042

Author: kernc

backtesting/_plotting.py

@@ -670,7 +670,11 @@ def __eq__(self, other):
                                                   code=_AUTOSCALE_JS_CALLBACK))
 
     figs = figs_above_ohlc + [fig_ohlc] + figs_below_ohlc
-    linked_crosshair = CrosshairTool(dimensions='both')
+    linked_crosshair = CrosshairTool(
+        dimensions='both', line_color='lightgrey',
+        overlay=(Span(dimension="width", line_dash="dotted", line_width=1),
+                 Span(dimension="height", line_dash="dotted", line_width=1)),
+    )
 
     for f in figs:
         if f.legend: