title: bokeh multiple figures with shared legend
body: <p>When following the <a href="http://bokeh.pydata.org/en/dev/docs/user_guide/layout.html" rel="noreferrer">bokeh docs</a> for making gridplots, if I use <code>figure</code>'s <code>legend</code> keyword argument I get a legend inside each subplot. How could I get a shared legend for the entire gridplot? Further, how could I get a shared legend between say 3 out of 4 of the subplots?</p>