[Python] How to get a discrete colour bar for plotly Scatter plot?

I have defined a list of 5 colours, which I use to colour the markers in the go.Scatter plot below.

I want the colour bar (legend) to show these discrete values, but the resulting colour bar stays white. See the white/empty bar titled "outputs" in the image.



Reproducible example


import numpy as np
import plotly.graph_objects as go

#Data for testing
outputs = np.random.choice([0, 1, 2, 3, 4], size=100)

#Figure
marker_palette=[
'rgb(39, 100, 25)', # ++
'rgb(156, 207, 100)', # +
'rgb(247, 247, 247)', # 0
'rgb(232, 150, 196)', # -
'rgb(142, 1, 82)', # --
]

scatter = go.Scatter(
x=np.random.randn(100),
y=np.random.randn(100),
mode='markers',
marker_color=[marker_palette[o] for o in outputs],
marker=dict(
# color=outputs, colorscale=marker_palette, #creates a continuous scale - not what I want
colorbar={
'title': 'outputs',
'tickmode': 'array',
'tickvals': np.unique(outputs),
'ticktext': ['++', '+', '0', '-', '--']
},
#/colorbar
),
#/marker
)
go.Figure(scatter)


I can get a continuous colour bar without any problems, but I want a discrete one.

I would prefer solutions using graph_objects as opposed to plotly.express, as I want to see how the internals get configured in your answer (though it's not a strict requirement).

How do I get a colour bar that shows the discrete colours?

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