Hello everyone,

I would like to add a node attribute `attribute`

to a graph after it has been created.

I tried using something like `graph.nodes.data['attribute'] = [1,2,3,4]`

. However, `graph.columns() `

does not reflect that change and as a result my program shows the error message

(BAD_COLUMN_NAME): Glyph refers to nonexistent column name. This could either be due to a misspelling or typo, or due to an expected

How do I add `attribute`

correctly AFTER the graph has been created? (meaning how do I get `graph.columns() `

to display my added attribute).

A small example for a graph (though without the error message) is

```
import numpy as np
import holoviews as hv
# set up the graph
node_indices = np.arange(3, dtype=np.int32)
source = np.zeros(3, dtype=np.int32)
target = node_indices
# create graph
x, y = [0, 1, 1], [0, 1, 2] # some coordinates
nodes = hv.Nodes((x, y, node_indices),)
graph = hv.Graph(((source, target), nodes))
# add an attribute after the graph has been created
graph.nodes.data["attribute"] = [1,2,3]
print(graph.nodes.data)
# result:
# x y index attribute
#0 0 0 0 1
#1 1 1 1 2
#2 1 2 2 3
print(graph.nodes.columns())
# result: OrderedDict([('x', array([0, 1, 1])), ('y', array([0, 1, 2])), ('index', array([0, 1, 2], dtype=int32))])
```

If you have any ideas, please let me know. Thanks in advance!