Fix Grid level alignment (#3957)

examples/user_guide/Plotting_with_Bokeh.ipynb

@@ -282,6 +282,50 @@
     "img.opts(data_aspect=0.5, responsive=True, title='scale both')"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Alignment\n",
+    "\n",
+    "The aligment of a plot in a row or column can be controlled using ``align`` option. It controls both the vertical alignment in a row and the horizontal alignment in a column and can be set to one of `'start'`, `'center'` or `'end'` (where `'start'` is the default)."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "#### Vertical"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "points = hv.Points(data).opts(axiswise=True)\n",
+    "img = hv.Image((xs, ys, xs[:, np.newaxis]*np.sin(ys*4)))\n",
+    "\n",
+    "img + points.opts(height=200, align='end')"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "#### Horizontal"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "(img.opts(axiswise=True, width=200, align='center') + points).cols(1)"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},

holoviews/plotting/bokeh/element.py

@@ -51,6 +51,9 @@ class ElementPlot(BokehPlot, GenericElementPlot):
         both 'pan' and 'box_zoom' are drag tools, so if both are
         listed only the last one will be active.""")
 
+    align = param.ObjectSelector(default=None, objects=['start', 'center', 'end'], doc="""
+        Alignment (vertical or horizontal) of the plot in a layout.""")
+
     border = param.Number(default=10, doc="""
         Minimum border around plot.""")
 
@@ -503,6 +506,7 @@ def _plot_properties(self, key, element):
                 self.callbacks.append(PlotSizeCallback(self, [stream], None))
 
         plot_props = {
+            'align':         self.align,
             'margin':        self.margin,
             'max_width':     self.max_width,
             'max_height':    self.max_height,

holoviews/plotting/bokeh/plot.py

@@ -631,6 +631,8 @@ def _create_subplots(self, layout, ranges):
                 kwargs['frame_width'] = width
             if height is not None:
                 kwargs['frame_height'] = height
+            if c == 0:
+                kwargs['align'] = 'end'
             if c == 0 and r != 0:
                 kwargs['xaxis'] = None
             if c != 0 and r == 0: