diff --git a/holoviews/core/data/ibis.py b/holoviews/core/data/ibis.py
index 441f99a803..537c67be5a 100644
--- a/holoviews/core/data/ibis.py
+++ b/holoviews/core/data/ibis.py
@@ -34,8 +34,11 @@ def has_rowid(cls):
 
     @classmethod
     def is_rowid_zero_indexed(cls, data):
-        from ibis.client import find_backends, validate_backends
-        (backend,) = validate_backends(list(find_backends(data)))
+        try:
+            from ibis.client import find_backends, validate_backends
+            (backend,) = validate_backends(list(find_backends(data)))
+        except Exception:
+            backend = data._find_backend()
         return type(backend).__module__ in cls.zero_indexed_backend_modules
 
     @classmethod
diff --git a/holoviews/core/data/xarray.py b/holoviews/core/data/xarray.py
index 8c14ce3f68..78ca711968 100644
--- a/holoviews/core/data/xarray.py
+++ b/holoviews/core/data/xarray.py
@@ -217,9 +217,17 @@ def retrieve_unit_and_label(dim):
             # not need to be canonicalized
             if any(len(da.coords[c].shape) > 1 for c in da.coords):
                 continue
-            undeclared = [
-                c for c in da.coords if c not in kdims and len(da[c].shape) == 1 and
-                da[c].shape[0] > 1]
+            undeclared = []
+            for c in da.coords:
+                if c in kdims or len(da[c].shape) != 1 or da[c].shape[0] <= 1:
+                    # Skip if coord is declared, represents irregular coordinates or is constant
+                    continue
+                elif all(d in kdims for d in da[c].dims):
+                    continue # Skip if coord is alias for another dimension
+                elif any(all(d in da[kd.name].dims for d in da[c].dims) for kd in kdims):
+                    # Skip if all the dims on the coord are present on another coord
+                    continue
+                undeclared.append(c)
             if undeclared:
                 raise DataError(
                     'The coordinates on the %r DataArray do not match the '
diff --git a/holoviews/core/util.py b/holoviews/core/util.py
index 32d9667a0d..3563df40ac 100644
--- a/holoviews/core/util.py
+++ b/holoviews/core/util.py
@@ -1569,17 +1569,27 @@ def is_param_method(obj, has_deps=False):
 def resolve_dependent_value(value):
     """Resolves parameter dependencies on the supplied value
 
-    Resolves parameter values, Parameterized instance methods and
-    parameterized functions with dependencies on the supplied value.
+    Resolves parameter values, Parameterized instance methods,
+    parameterized functions with dependencies on the supplied value,
+    including such parameters embedded in a list or tuple.
 
     Args:
        value: A value which will be resolved
 
     Returns:
-       A new dictionary where any parameter dependencies have been
+       A new value where any parameter dependencies have been
        resolved.
     """
     range_widget = False
+    if isinstance(value, list):
+        value = [resolve_dependent_value(v) for v in value]
+    elif isinstance(value, tuple):
+        value = tuple(resolve_dependent_value(v) for v in value)
+    elif isinstance(value, dict):
+        value = {
+            resolve_dependent_value(k): resolve_dependent_value(v) for k, v in value.items()
+        }
+
     if 'panel' in sys.modules:
         from panel.widgets import RangeSlider, Widget
         range_widget = isinstance(value, RangeSlider)
@@ -1614,7 +1624,7 @@ def resolve_dependent_kwargs(kwargs):
        kwargs (dict): A dictionary of keyword arguments
 
     Returns:
-       A new dictionary with where any parameter dependencies have been
+       A new dictionary where any parameter dependencies have been
        resolved.
     """
     return {k: resolve_dependent_value(v) for k, v in kwargs.items()}
@@ -2294,3 +2304,30 @@ def cast_array_to_int64(array):
             category=FutureWarning,
         )
         return array.astype('int64')
+
+
+def flatten(line):
+    """
+    Flatten an arbitrarily nested sequence.
+
+    Inspired by: pd.core.common.flatten
+
+    Parameters
+    ----------
+    line : sequence
+        The sequence to flatten
+
+    Notes
+    -----
+    This only flattens list, tuple, and dict sequences.
+
+    Returns
+    -------
+    flattened : generator
+    """
+
+    for element in line:
+        if any(isinstance(element, tp) for tp in (list, tuple, dict)):
+            yield from flatten(element)
+        else:
+            yield element
diff --git a/holoviews/operation/datashader.py b/holoviews/operation/datashader.py
index c5fe4706bf..154130f3e6 100644
--- a/holoviews/operation/datashader.py
+++ b/holoviews/operation/datashader.py
@@ -2,7 +2,7 @@
 
 import warnings
 
-from collections import Callable
+from collections.abc import Callable
 from functools import partial
 
 import param
diff --git a/holoviews/plotting/bokeh/element.py b/holoviews/plotting/bokeh/element.py
index d13655ea4c..32931d8f90 100644
--- a/holoviews/plotting/bokeh/element.py
+++ b/holoviews/plotting/bokeh/element.py
@@ -2,6 +2,8 @@
 
 import sys
 import warnings
+
+from itertools import chain
 from types import FunctionType
 
 import param
@@ -2317,13 +2319,13 @@ def _get_factors(self, overlay, ranges):
             if el is not None:
                 elranges = util.match_spec(el, ranges)
                 xfs, yfs = sp._get_factors(el, elranges)
-                xfactors.append(xfs)
-                yfactors.append(yfs)
-        if xfactors:
-            xfactors = np.concatenate(xfactors)
-        if yfactors:
-            yfactors = np.concatenate(yfactors)
-        return util.unique_array(xfactors), util.unique_array(yfactors)
+                if len(xfs):
+                    xfactors.append(xfs)
+                if len(yfs):
+                    yfactors.append(yfs)
+        xfactors = list(util.unique_iterator(chain(*xfactors)))
+        yfactors = list(util.unique_iterator(chain(*yfactors)))
+        return xfactors, yfactors
 
 
     def _get_axis_dims(self, element):
diff --git a/holoviews/plotting/mpl/element.py b/holoviews/plotting/mpl/element.py
index 73c0faae09..f308a10266 100644
--- a/holoviews/plotting/mpl/element.py
+++ b/holoviews/plotting/mpl/element.py
@@ -174,7 +174,7 @@ def _finalize_axis(self, key, element=None, title=None, dimensions=None, ranges=
                 if self.logy:
                     axis.set_yscale('log')
 
-                if not isinstance(self.projection, str) and self.projection == '3d':
+                if not (isinstance(self.projection, str) and self.projection == '3d'):
                     self._set_axis_position(axis, 'x', self.xaxis)
                     self._set_axis_position(axis, 'y', self.yaxis)
 
@@ -330,7 +330,7 @@ def _set_axis_limits(self, axis, view, subplots, ranges):
         coords = [coord if isinstance(coord, np.datetime64) or np.isreal(coord) else np.NaN for coord in extents]
         coords = [date2num(util.dt64_to_dt(c)) if isinstance(c, np.datetime64) else c
                   for c in coords]
-        if isinstance(self.projection, str) and self.projection == '3d' or len(extents) == 6:
+        if (isinstance(self.projection, str) and self.projection == '3d') or len(extents) == 6:
             l, b, zmin, r, t, zmax = coords
             if self.invert_zaxis or any(p.invert_zaxis for p in subplots):
                 zmin, zmax = zmax, zmin
diff --git a/holoviews/streams.py b/holoviews/streams.py
index 95e2c29516..9512356c15 100644
--- a/holoviews/streams.py
+++ b/holoviews/streams.py
@@ -770,7 +770,7 @@ def hashkey(self):
         for p in self.parameters:
             pkey = (p.owner, p.name)
             pname = self._rename.get(pkey, p.name)
-            key = ' '.join([p.owner.name, pname])
+            key = ' '.join([str(id(p.owner)), pname])
             if self._rename.get(pkey, True) is not None:
                 hashkey[key] = getattr(p.owner, p.name)
         hashkey['_memoize_key'] = self._memoize_counter
diff --git a/holoviews/tests/core/data/testxarrayinterface.py b/holoviews/tests/core/data/testxarrayinterface.py
index b37eb36c75..be80aa0f1a 100644
--- a/holoviews/tests/core/data/testxarrayinterface.py
+++ b/holoviews/tests/core/data/testxarrayinterface.py
@@ -60,6 +60,14 @@ def get_multi_dim_irregular_dataset(self):
                                 'time': pd.date_range('2014-09-06', periods=3),
                                 'reference_time': pd.Timestamp('2014-09-05')})
 
+    def test_ignore_dependent_dimensions_if_not_specified(self):
+        coords = OrderedDict([('time', [0, 1]), ('lat', [0, 1]), ('lon', [0, 1])])
+        da = xr.DataArray(np.arange(8).reshape((2, 2, 2)),
+                          coords, ['time', 'lat', 'lon']).assign_coords(
+                              lat1=xr.DataArray([2,3], dims=['lat']))
+        assert Dataset(da, ['time', 'lat', 'lon'], vdims='value').kdims == ['time', 'lat', 'lon']
+        assert Dataset(da, ['time', 'lat1', 'lon'], vdims='value').kdims == ['time', 'lat1', 'lon']
+
     def test_xarray_dataset_irregular_shape(self):
         ds = Dataset(self.get_multi_dim_irregular_dataset())
         shape = ds.interface.shape(ds, gridded=True)
diff --git a/holoviews/tests/core/testapply.py b/holoviews/tests/core/testapply.py
index ea1ae06ecc..fd4d6401ba 100644
--- a/holoviews/tests/core/testapply.py
+++ b/holoviews/tests/core/testapply.py
@@ -1,12 +1,14 @@
 import numpy as np
+import pandas as pd
 import param
 
-from panel.widgets import TextInput
+from panel.widgets import RadioButtonGroup, TextInput
 
+from holoviews import Dataset, util
 from holoviews.core.spaces import DynamicMap, HoloMap
-from holoviews.element import Image, Curve
+from holoviews.element import Curve, Image
 from holoviews.element.comparison import ComparisonTestCase
-from holoviews.streams import Params, ParamMethod
+from holoviews.streams import ParamMethod, Params
 
 
 class ParamClass(param.Parameterized):
@@ -275,3 +277,18 @@ def test_dmap_apply_dynamic_with_param_method(self):
         self.assertEqual(applied[1], self.dmap[1].relabel('Test!'))
         pinst.label = 'Another label'
         self.assertEqual(applied[1], self.dmap[1].relabel('Another label!'))
+
+
+def test_nested_widgets():
+    df = pd._testing.makeDataFrame()
+    column = RadioButtonGroup(value="A", options=list("ABC"))
+    ds = Dataset(df)
+    transform = util.transform.df_dim("*").groupby(["D", column]).mean()
+
+    params = list(transform.params.values())
+    assert len(params) == 1
+    assert params[0] == column.param.value
+
+    df1 = transform.apply(ds, keep_index=True, compute=False)
+    df2 = df.groupby(["D", "A"]).mean()
+    pd.testing.assert_frame_equal(df1, df2)
diff --git a/holoviews/tests/plotting/bokeh/testoverlayplot.py b/holoviews/tests/plotting/bokeh/testoverlayplot.py
index 5f9b15a4d4..8599c8bb94 100644
--- a/holoviews/tests/plotting/bokeh/testoverlayplot.py
+++ b/holoviews/tests/plotting/bokeh/testoverlayplot.py
@@ -3,7 +3,7 @@
 
 from holoviews.core import NdOverlay, HoloMap, DynamicMap, Overlay
 from holoviews.core.options import Cycle
-from holoviews.element import Curve, Points, ErrorBars, Scatter, Text, VLine
+from holoviews.element import Bars, Curve, ErrorBars, HLine, Points, Scatter, Text, VLine
 from holoviews.streams import Stream, Tap
 from holoviews.util import Dynamic
 
@@ -11,7 +11,7 @@
 from .testplot import TestBokehPlot, bokeh_renderer
 
 try:
-    from bokeh.models import FixedTicker, HoverTool, FactorRange, Range1d
+    from bokeh.models import FixedTicker, HoverTool, FactorRange, Span, Range1d
 except:
     pass
 
@@ -192,6 +192,16 @@ def test_points_errorbars_text_ndoverlay_categorical_xaxis(self):
         for xs, factor in zip(error_plot.handles['source'].data['base'], factors):
             self.assertEqual(factor, xs)
 
+    def test_overlay_categorical_two_level(self):
+        bars = Bars([('A', 'a', 1), ('B', 'b', 2), ('A', 'b', 3), ('B', 'a', 4)],
+                    kdims=['Upper', 'Lower'])
+
+        plot = bokeh_renderer.get_plot(bars * HLine(2))
+        x_range = plot.handles['x_range']
+        assert isinstance(x_range, FactorRange)
+        assert x_range.factors == [('A', 'a'), ('A', 'b'), ('B', 'a'), ('B', 'b')]
+        assert isinstance(plot.state.renderers[-1], Span)
+
     def test_points_errorbars_text_ndoverlay_categorical_xaxis_invert_axes(self):
         overlay = NdOverlay({i: Points(([chr(65+i)]*10,np.random.randn(10)))
                              for i in range(5)})
diff --git a/holoviews/tests/plotting/bokeh/testserver.py b/holoviews/tests/plotting/bokeh/testserver.py
index c85f7f8998..1ff9ad0d98 100644
--- a/holoviews/tests/plotting/bokeh/testserver.py
+++ b/holoviews/tests/plotting/bokeh/testserver.py
@@ -1,8 +1,6 @@
 import time
-import threading
 
 from unittest import SkipTest
-from threading import Event
 
 import param
 
@@ -14,21 +12,18 @@
 from holoviews.streams import Stream, RangeXY, PlotReset
 
 try:
-    from bokeh.application.handlers import FunctionHandler
-    from bokeh.application import Application
     from bokeh.client import pull_session
     from bokeh.document import Document
     from bokeh.io.doc import curdoc, set_curdoc
     from bokeh.models import ColumnDataSource
-    from bokeh.server.server import Server
 
     from holoviews.plotting.bokeh.callbacks import (
         Callback, RangeXYCallback, ResetCallback
     )
     from holoviews.plotting.bokeh.renderer import BokehRenderer
     from panel.widgets import DiscreteSlider, FloatSlider
-    from panel.io.server import StoppableThread
     from panel.io.state import state
+    from panel import serve
     bokeh_renderer = BokehRenderer.instance(mode='server')
 except:
     bokeh_renderer = None
@@ -107,78 +102,39 @@ def setUp(self):
         if not bokeh_renderer:
             raise SkipTest("Bokeh required to test plot instantiation")
         Store.current_backend = 'bokeh'
-        self._loaded = Event()
         self._port = None
-        self._thread = None
-        self._server = None
 
     def tearDown(self):
         Store.current_backend = self.previous_backend
         Callback._callbacks = {}
-        if self._thread is not None:
-            try:
-                self._thread.stop()
-            except:
-                pass
-        state._thread_id = None
-        if self._server is not None:
-            try:
-                self._server.stop()
-            except:
-                pass
+        state.kill_all_servers()
         time.sleep(1)
 
-    def _launcher(self, obj, threaded=False, io_loop=None):
-        if io_loop:
-            io_loop.make_current()
-        launched = []
-        def modify_doc(doc):
-            bokeh_renderer(obj, doc=doc)
-            launched.append(True)
-        handler = FunctionHandler(modify_doc)
-        app = Application(handler)
-        server = Server({'/': app}, port=0, io_loop=io_loop)
-        server.start()
-        self._port = server.port
-        self._server = server
-        if threaded:
-            server.io_loop.add_callback(self._loaded.set)
-            thread = threading.current_thread()
-            state._thread_id = thread.ident if thread else None
-            io_loop.start()
-        else:
-            url = "http://localhost:" + str(server.port) + "/"
-            session = pull_session(session_id='Test', url=url, io_loop=server.io_loop)
-            self.assertTrue(len(launched)==1)
-            return session, server
-        return None, server
-
-    def _threaded_launcher(self, obj):
-        from tornado.ioloop import IOLoop
-        io_loop = IOLoop()
-        thread = StoppableThread(target=self._launcher, io_loop=io_loop,
-                                 args=(obj, True, io_loop))
-        thread.setDaemon(True)
-        thread.start()
-        self._loaded.wait()
-        self._thread = thread
-        return self.session
+    def _launcher(self, obj, threaded=True, port=6001):
+        self._port = port
+        server = serve(obj, threaded=threaded, show=False, port=port)
+        time.sleep(0.5)
+        return server, self.session
 
     @property
     def session(self):
         url = "http://localhost:" + str(self._port) + "/"
         return pull_session(session_id='Test', url=url)
-
+    
     def test_launch_simple_server(self):
         obj = Curve([])
-        self._launcher(obj)
+        server, _ = self._launcher(obj, port=6001)
+        server.stop()
 
     def test_launch_server_with_stream(self):
-        obj = Curve([])
-        stream = RangeXY(source=obj)
+        el = Curve([])
+        stream = RangeXY(source=el)
 
-        _, server = self._launcher(obj)
-        cb = bokeh_renderer.last_plot.callbacks[0]
+        obj, _ = bokeh_renderer._validate(el, None)
+        server, _ = self._launcher(obj, port=6002)
+        [(plot, _)] = obj._plots.values()
+
+        cb = plot.callbacks[0]
         self.assertIsInstance(cb, RangeXYCallback)
         self.assertEqual(cb.streams, [stream])
         x_range = bokeh_renderer.last_plot.handles['x_range']
@@ -195,15 +151,15 @@ def test_launch_server_with_complex_plot(self):
         static = Polygons([]) * Path([]) * Curve([])
         layout = overlay + static
 
-        _, server = self._launcher(layout)
+        server, _ = self._launcher(layout, port=6003)
         server.stop()
 
     def test_server_dynamicmap_with_dims(self):
         dmap = DynamicMap(lambda y: Curve([1, 2, y]), kdims=['y']).redim.range(y=(0.1, 5))
         obj, _ = bokeh_renderer._validate(dmap, None)
-        session = self._threaded_launcher(obj)
+        server, session = self._launcher(obj, port=6004)
         [(plot, _)] = obj._plots.values()
-        [(doc, _)] = obj.layout._documents.items()
+        [(doc, _)] = obj._documents.items()
 
         cds = session.document.roots[0].select_one({'type': ColumnDataSource})
         self.assertEqual(cds.data['y'][2], 0.1)
@@ -214,13 +170,14 @@ def run():
         time.sleep(1)
         cds = self.session.document.roots[0].select_one({'type': ColumnDataSource})
         self.assertEqual(cds.data['y'][2], 3.1)
+        server.stop()
 
     def test_server_dynamicmap_with_stream(self):
         stream = Stream.define('Custom', y=2)()
         dmap = DynamicMap(lambda y: Curve([1, 2, y]), kdims=['y'], streams=[stream])
         obj, _ = bokeh_renderer._validate(dmap, None)
-        session = self._threaded_launcher(obj)
-        [(doc, _)] = obj.layout._documents.items()
+        server, session = self._launcher(obj, port=6005)
+        [(doc, _)] = obj._documents.items()
 
         cds = session.document.roots[0].select_one({'type': ColumnDataSource})
         self.assertEqual(cds.data['y'][2], 2)
@@ -230,14 +187,15 @@ def run():
         time.sleep(1)
         cds = self.session.document.roots[0].select_one({'type': ColumnDataSource})
         self.assertEqual(cds.data['y'][2], 3)
+        server.stop()
 
     def test_server_dynamicmap_with_stream_dims(self):
         stream = Stream.define('Custom', y=2)()
         dmap = DynamicMap(lambda x, y: Curve([x, 1, y]), kdims=['x', 'y'],
                           streams=[stream]).redim.values(x=[1, 2, 3])
         obj, _ = bokeh_renderer._validate(dmap, None)
-        session = self._threaded_launcher(obj)
-        [(doc, _)] = obj.layout._documents.items()
+        server, session = self._launcher(obj, port=6006)
+        [(doc, _)] = obj._documents.items()
 
         orig_cds = session.document.roots[0].select_one({'type': ColumnDataSource})
         self.assertEqual(orig_cds.data['y'][2], 2)
@@ -256,3 +214,4 @@ def run():
         time.sleep(1)
         cds = self.session.document.roots[0].select_one({'type': ColumnDataSource})
         self.assertEqual(cds.data['y'][0], 3)
+        server.stop()
diff --git a/holoviews/tests/plotting/matplotlib/testelementplot.py b/holoviews/tests/plotting/matplotlib/testelementplot.py
index 7f7104b613..f7be9d6d39 100644
--- a/holoviews/tests/plotting/matplotlib/testelementplot.py
+++ b/holoviews/tests/plotting/matplotlib/testelementplot.py
@@ -59,6 +59,14 @@ def test_element_font_scaling_fontsize_override_specific(self):
         self.assertEqual(ax.xaxis._major_tick_kw['labelsize'], 24)
         self.assertEqual(ax.yaxis._major_tick_kw['labelsize'], 20)
 
+    def test_element_no_xaxis_yaxis(self):
+        element = Curve(range(10)).options(xaxis=None, yaxis=None)
+        axes = mpl_renderer.get_plot(element).handles['axis']
+        xaxis = axes.get_xaxis()
+        yaxis = axes.get_yaxis()
+        self.assertEqual(xaxis.get_visible(), False)
+        self.assertEqual(yaxis.get_visible(), False)
+
     def test_element_xlabel(self):
         element = Curve(range(10)).options(xlabel='custom x-label')
         axes = mpl_renderer.get_plot(element).handles['axis']
diff --git a/holoviews/tests/teststreams.py b/holoviews/tests/teststreams.py
index 98bf2a50be..2538889bd6 100644
--- a/holoviews/tests/teststreams.py
+++ b/holoviews/tests/teststreams.py
@@ -5,6 +5,7 @@
 from unittest import SkipTest
 
 import param
+from panel.widgets import IntSlider
 
 from holoviews.core.spaces import DynamicMap
 from holoviews.core.util import LooseVersion, pd
@@ -272,7 +273,7 @@ def test_param_stream_action(self):
         def subscriber(**kwargs):
             values.append(kwargs)
             self.assertEqual(set(stream.hashkey),
-                             {'%s action' % inner.name, '_memoize_key'})
+                             {'%s action' % id(inner), '_memoize_key'})
 
         stream.add_subscriber(subscriber)
         inner.action(inner)
@@ -288,7 +289,7 @@ def subscriber(**kwargs):
             values.append(kwargs)
             self.assertEqual(
                 set(stream.hashkey),
-                {'%s action' % inner.name, '%s x' % inner.name, '_memoize_key'})
+                {'%s action' % id(inner), '%s x' % id(inner), '_memoize_key'})
 
         stream.add_subscriber(subscriber)
         inner.action(inner)
@@ -296,6 +297,18 @@ def subscriber(**kwargs):
         self.assertEqual(values, [{'action': inner.action, 'x': 0}])
 
 
+    def test_params_no_names(self):
+        a = IntSlider()
+        b = IntSlider()
+        p = Params(parameters=[a.param.value, b.param.value])
+        assert len(p.hashkey) == 3  # the two widgets + _memoize_key
+
+    def test_params_identical_names(self):
+        a = IntSlider(name="Name")
+        b = IntSlider(name="Name")
+        p = Params(parameters=[a.param.value, b.param.value])
+        assert len(p.hashkey) == 3  # the two widgets + _memoize_key
+
 
 class TestParamMethodStream(ComparisonTestCase):
 
@@ -501,7 +514,7 @@ def test_dynamicmap_param_method_action_param(self):
         def subscriber(**kwargs):
             values.append(kwargs)
             self.assertEqual(set(stream.hashkey),
-                             {'%s action' % inner.name, '_memoize_key'})
+                             {'%s action' % id(inner), '_memoize_key'})
 
         stream.add_subscriber(subscriber)
         inner.action(inner)
@@ -521,7 +534,7 @@ def subscriber(**kwargs):
             values.append(kwargs)
             self.assertEqual(
                 set(stream.hashkey),
-                {'%s action' % inner.name, '%s x' % inner.name, '_memoize_key'})
+                {'%s action' % id(inner), '%s x' % id(inner), '_memoize_key'})
 
         stream.add_subscriber(subscriber)
         stream.add_subscriber(lambda **kwargs: dmap[()])
diff --git a/holoviews/util/transform.py b/holoviews/util/transform.py
index fef557b145..89033a06b8 100644
--- a/holoviews/util/transform.py
+++ b/holoviews/util/transform.py
@@ -10,8 +10,7 @@
 
 from ..core.data import PandasInterface
 from ..core.dimension import Dimension
-from ..core.util import basestring, pd, resolve_dependent_value, unique_iterator
-
+from ..core.util import basestring, flatten, pd, resolve_dependent_value, unique_iterator
 
 def _maybe_map(numpy_fn):
     def fn(values, *args, **kwargs):
@@ -332,6 +331,7 @@ def params(self):
         params = {}
         for op in self.ops:
             op_args = list(op['args'])+list(op['kwargs'].values())
+            op_args = flatten(op_args)
             for op_arg in op_args:
                 if 'panel' in sys.modules:
                     from panel.widgets.base import Widget