Hi, I believe I have encountered this issue, but want to check. Cross-posted on SO here.

Let’s say I want to add horizontal lines on my chart which correspond to the median value of some group, or a standard deviation band. For example, the median number of apples sold. (Thanks to previous posters who helped with this; you’ll recognize this toy problem.) Is this possible in Bokeh? Is it possible in any Python plotting library?

```
import pandas as pd
from bokeh.io import output_notebook, show, reset_output
from bokeh.palettes import Spectral5, Turbo256
from bokeh.plotting import figure
from bokeh.transform import factor_cmap
from bokeh.models import Band, Span, FactorRange, ColumnDataSource
fruits = ['Apples', 'Pears']
years = ['2015', '2016']
data = {'fruits' : fruits,
'2015' : [2, 1],
'2016' : [5, 3]}
fruit_df = pd.DataFrame(data).set_index("fruits")
tidy_df = (pd.DataFrame(data)
.melt(id_vars=["fruit"], var_name="year")
.assign(fruit_year=lambda df: list(zip(df['fruit'], df['year'])))
.set_index('fruit_year'))
```

Now on to making the figure…

```
p = figure(x_range=FactorRange(factors=tidy_df.index.unique()),
plot_height=400,
plot_width=400,
tooltips=[('Fruit', '@fruit'), # first string is user-defined; second string must refer to a column
('Year', '@year'),
('Value', '@value')])
cds = ColumnDataSource(tidy_df)
index_cmap = factor_cmap("fruit",
Spectral5[:2],
factors=sorted(tidy_df["fruit"].unique())) # this is a reference back to the dataframe
p.circle(x='fruit_year',
y='value',
size=20,
source=cds,
fill_color=index_cmap,
line_color=None,
)
# how do I add a median just to one categorical section?
median = Span(location=tidy_df.loc[tidy_df["fruit"] == "Apples", "value"].median(), # median value for Apples
#dimension='height',
line_color='red',
line_dash='dashed',
line_width=1.0
)
p.add_layout(median)
# how do I add this standard deviation(ish) band to just the Apples or Pears section?
band = Band(
base='fruit_year',
lower=2,
upper=4,
source=cds,
)
p.add_layout(band)
show(p)
```

Output:

Here’s what I want, done quickly in Tableau:

Is there a way to make this happen? Thank you!