allow provision of X coordinates only for Sankey charts

This is somewhat of a duplicate of #960 and #1437 but slightly more hopeful :)
The x and y arguments of the nodes dict do successfully control node location, however -- it appears that both arguments must be supplied.

import dash
import dash_core_components as dcc
import dash_html_components as html
f#rom dash.dependencies import Output, Input, State
import plotly.graph_objs as go
#import pandas as pd

app = dash.Dash()
app.layout = html.Div([
    dcc.Graph(
        id = 'wut',
        figure={
            'data' : [
                dict(
                    type='sankey',
                    #domain = 
                    arrangement = 'fixed',
                        node = dict(
                            pad = 15,
                            thickness = 20,
                            line = dict(color = "black", width = 0.5),
                            label = ["A1", "A2", "B1", "B2", "C1", "C2"],
                            x = [0.1, 0.2, 0.24, 0.7, 0.8, .9],
                            y = [0, .2, .3, .4, .5, .6],
                            color = "blue"
                    ),
            link = dict(
                source = [0, 1, 0, 2, 3, 3], # indices correspond to labels, eg A1, A2, A2, B1, ...
                target = [2, 3, 3, 4, 4, 5],
                value = [8, 4, 2, 8, 4, 2]
            ))        
        ]}   
)])

if __name__ == '__main__':
    app.run_server(debug=True)

VS.

import dash
import dash_core_components as dcc
import dash_html_components as html
f#rom dash.dependencies import Output, Input, State
import plotly.graph_objs as go
#import pandas as pd

app = dash.Dash()
app.layout = html.Div([
    dcc.Graph(
        id = 'wut',
        figure={
            'data' : [
                dict(
                    type='sankey',
                    #domain = 
                    arrangement = 'fixed',
                        node = dict(
                            pad = 15,
                            thickness = 20,
                            line = dict(color = "black", width = 0.5),
                            label = ["A1", "A2", "B1", "B2", "C1", "C2"],
                            ###x = [0.1, 0.2, 0.24, 0.7, 0.8, .9],
                            y = [0, .2, .3, .4, .5, .6],
                            color = "blue"
                    ),
            link = dict(
                source = [0, 1, 0, 2, 3, 3], # indices correspond to labels, eg A1, A2, A2, B1, ...
                target = [2, 3, 3, 4, 4, 5],
                value = [8, 4, 2, 8, 4, 2]
            ))        
        ]}   
)])

if __name__ == '__main__':
    app.run_server(debug=True)

No files or tests are named. Reproduce the two Sankey examples from the issue, then trace where Sankey node coordinates are handled to determine why x currently requires y. Done means a Sankey chart with arrangement set to fixed accepts x coordinates without y and preserves the documented behavior.