title: Bokeh remove space between elements in gridplot
body: <p>I have a Bokeh app (<code>bokeh==1.0.1</code>) where I use gridplot to render a slider widgetbox and several figures. I am using <code>sizing_mode='stretch_both'</code>. How can I eliminate the white space between the widget row and the rest of the figure panels? Here is a screenshot:</p>
  
  <p><a href="https://i.stack.imgur.com/CRzgR.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/CRzgR.png" alt="enter image description here"></a> </p>
  
  <p>I create a layout for the widget as:</p>
  
  <pre><code>blank_button = bkm.widgets.RadioButtonGroup()
  blank_wb = widgetbox(blank_button, width=50, height=100, sizing_mode='fixed') # placeholder for space
  date_slider_wb = widgetbox(date_slider, width=400, height=100, sizing_mode='fixed')
  apply_button = bkm.widgets.Button(label="APPLY")
  apply_wb = widgetbox(apply_button, width=100, height=100, sizing_mode='fixed')
  
  hor_layout_date_slider = (
      row(
        column(blank_wb),
        column(date_slider_wb),
        column(apply_wb),
      )
  )
  </code></pre>
  
  <p>Then I include <code>hor_layout_date_slider</code> in a <code>gridplot</code> as:</p>
  
  <pre><code>grid = gridplot(
    children = [
      hor_layout_date_slider,
      airtemp_fig,
      windspeed_fig,
      winddir_fig,
      interval_fig,
      precip_fig,
      pressure_fig
      ],
      ncols=1,
      sizing_mode='stretch_both',
      merge_tools=False,
  )
  
  curdoc().add_root(grid)
  </code></pre>
  
  <p>I am using the bokeh server and rendering the bokeh document in a single div in my html template as:</p>
  
  <pre><code>  &lt;div class="placeholderbokehapp rounded" id="tbc-id"&gt;
  
  &lt;script src="http://localhost:5858/stormtracker_bokehapp/autoload.js?bokeh-autoload-element=1000&amp;bokeh-app-path=/stormtracker_bokehapp&amp;bokeh-absolute-url=http://localhost:5858/stormtracker_bokehapp&amp;resources=none" id="1000"&gt;&lt;/script&gt;
    &lt;/div&gt;
  </code></pre>
  
  <p>As an attempted hack solution, I have been able to pass custom css rules to define the height of the div containing <code>hor_layout_date_slider</code>, but there is still a "place holder" for this element that persists (which I cannot access with inspect elements).</p>
  
  <p>I have also tried using a simple widgetbox containing only the slider (without defining height and width and using <code>sizing_mode='stretch_both'</code>), instead of the full <code>hor_layout_date_slider</code> as defined above. However, this results in the same white space following the slider element.</p>
  
  <p>Strangely enough, this problem does not occur with <code>sizing_mode='scale_width'</code> (the slider is tight in the layout).</p>
  
  <p>Is there a Bokeh setting I am not aware of to control the spacing in this layout when using <code>sizing_mode='stretch_both'</code>?</p>
  
  <p><strong>UPDATE:</strong></p>
  
  <p>If I add the widget and the grid separately as:</p>
  
  <pre><code>curdoc().add_root(hor_layout_date_slider)
  curdoc().add_root(grid)
  </code></pre>
  
  <p>The widget is then rendered underneath the first figure panel (you can see part of the slider widget showing through in the screenshot below).</p>
  
  <p><a href="https://i.stack.imgur.com/OLXcW.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/OLXcW.png" alt="enter image description here"></a></p>