Add one specific datapoint marker to boxplot or violinplot (using holoviews / hvplot)

Question

I made a violinplot of 1 column in my dataframe.
Now I would like to show where one specific datapoint (outlier) lies in comparison with that violinplot.

Below I have created some sample data and I would to plot 1 specific outlier, for example a datapoint with value 20.
How do I add this datapoint with a marker to my plot?

The problem is I know the value of 20 which is the x-axis, but I don't know the value of the y-axis, it seems to be 'anonymous'. I tried to overlay a scatterplot with point (None, 20), but that didn't work.

Example code:

# import libaries
import numpy as np
import pandas as pd
import hvplot
import hvplot.pandas
import holoviews as hv
hv.extension('bokeh')

# create sample_data
sample_data = np.random.poisson(lam=50, size=100)
df = pd.DataFrame(sample_data, columns=['col1'])

# create violin plot with hvplot:
violin_plot = df.hvplot.violin(ylim=(-5, 100), invert=True)

# or create it with holoviews:
violin_plot = hv.Violin(df, vdims=['col1'])
violin_plot.opts(ylim=(-5, 100), invert_axes=True, width=500)

Current plot without the specific datapoint:

Answer 1

You need to overlay a scatter plot with the value of your datapoint on your violin plot.
This is basically putting the scatter plot on top of your violin plot and this can be done with the * symbol.

The y-axis here looks 'anonymous' but the key lies in the fact that it actually has value '', so it's an empty string.
So if you want to plot value 20, then your datapoint is ('', 20).

Solution code:

# x-axis is an empty string, so create a scatter plot of 1 datapoint with value ('', 80)
scatter_point = hv.Scatter(('', 20)).opts(color='red', size=10)

# put your scatter plot with 1 datapoint on top of your violin plot using the * symbol
violin_plot * scatter_point

Resulting plot: