Exception: "None of the available storage backends were able to support the supplied data format."

I try to use hvplot.networkx from hvplot version 0.5.2 with bokeh to visualize a network graph.

This is code that works: a simple graph is created using networkx, and some attributes are added to the graph nodes, then hvplot is used to show the graph and show hover tooltips from the attributes:
```
import networkx as nx
import hvplot.networkx as hvnx

nodes = [1,2,3,4,5,6,7,8,9,10]
edges = [(1,2), (1,3), (1,4), (4,5), (4,3), (8,9), (8,7), (7,6)]

nxg1 = nx.Graph()
nxg1.add_nodes_from(nodes)
nxg1.add_edges_from(edges)
pos = nx.spring_layout(nxg1)

# some attributes for the nodes
for node in nxg1.nodes:
nxg1.nodes[node]["label"] = f"Node{node}"
nxg1.nodes[node]["desc"] = "some description"

hvnx.draw(nxg1, pos, hover_cols="all")
```

The following code throws the exception: here the node has an attribute "x" (or "y", but "z" is ok):
```
import networkx as nx
import hvplot.networkx as hvnx

nodes = [1,2,3,4,5,6,7,8,9,10]
edges = [(1,2), (1,3), (1,4), (4,5), (4,3), (8,9), (8,7), (7,6)]

nxg1 = nx.Graph()
nxg1.add_nodes_from(nodes)
nxg1.add_edges_from(edges)
pos = nx.spring_layout(nxg1)

# some attributes for the nodes
for node in nxg1.nodes:
nxg1.nodes[node]["label"] = f"Node{node}"
nxg1.nodes[node]["desc"] = "some description"
nxg1.nodes[node]["x"] = pos[node][0]
nxg1.nodes[node]["y"] = pos[node][1]
hvnx.draw(nxg1, pos, hover_cols="all")
```
Exception:
```
DataError Traceback (most recent call last)
in
16 nxg1.nodes[node]["x"] = pos[node][0]
17 nxg1.nodes[node]["x"] = pos[node][1]
---> 18 hvnx.draw(nxg1, pos, hover_cols="all")

~/software/anaconda/envs/polads/lib/python3.6/site-packages/hvplot/networkx.py in draw(G, pos, **kwargs)
223
224 # Construct Graph object
--> 225 g = _from_networkx(G, pos, **params)
226
227 if 'nodelist' in kwargs:

~/software/anaconda/envs/polads/lib/python3.6/site-packages/hvplot/networkx.py in _from_networkx(G, positions, nodes, cls, **kwargs)
107 dim = col
108 vdims.append(dim)
--> 109 nodes = cls.node_type(node_data, vdims=vdims)
110
111 # Construct graph

~/software/anaconda/envs/polads/lib/python3.6/site-packages/holoviews/core/data/__init__.py in __init__(self, data, kdims, vdims, **kwargs)
337 validate_vdims = kwargs.pop('_validate_vdims', True)
338 initialized = Interface.initialize(type(self), data, kdims, vdims,
--> 339 datatype=kwargs.get('datatype'))
340 (data, self.interface, dims, extra_kws) = initialized
341 super(Dataset, self).__init__(data, **dict(kwargs, **dict(dims, **extra_kws)))

~/software/anaconda/envs/polads/lib/python3.6/site-packages/holoviews/core/data/interface.py in initialize(cls, eltype, data, kdims, vdims, datatype)
290 error = ' '.join([error, priority_error])
291 raise six.reraise(DataError, DataError(error, intfc), sys.exc_info()[2])
--> 292 raise DataError(error)
293
294 return data, interface, dims, extra_kws

DataError: None of the available storage backends were able to support the supplied data format.
```

Why is "x" or "y" not usable as an attribute, are there other names that would not work and if there are reserved names, could the error message be more informative?