How to place arrows between multiple scatter points

Question

The code below produce this graph. I wonder if there is a way to make the lines between value1 and value2 into arrows, pointing in the direction of 1 to 2, from blue to green (In this case none of blues is lower than the greens).

import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
 
# Create a dataframe
value1=np.random.uniform(size=20)
value2=value1+np.random.uniform(size=20)/4
df = pd.DataFrame({'group':list(map(chr, range(65, 85))), 'value1':value1 , 'value2':value2 })
 
# Reorder it following the values of the first value:
ordered_df = df.sort_values(by='value1')
my_range=range(1,len(df.index)+1)
 
# The horizontal plot is made using the hline function
plt.hlines(y=my_range, xmin=ordered_df['value1'], xmax=ordered_df['value2'], color='grey', alpha=0.4)
plt.scatter(ordered_df['value1'], my_range, color='skyblue', alpha=1, label='value1')
plt.scatter(ordered_df['value2'], my_range, color='green', alpha=0.4 , label='value2')
plt.legend()
 
# Add title and axis names
plt.yticks(my_range, ordered_df['group'])
plt.title("Comparison of the value 1 and the value 2", loc='left')
plt.xlabel('Value of the variables')
plt.ylabel('Group')

# Show the graph
plt.show()

Answer 1

• The best option for multiple arrows is matplotlib.pyplot.quiver, because it accepts an array or dataframe of locations, unlike matplotlib.pyplot.arrow, which only accepts a single value.
  • Since the y-axis labels are defined by 'group', which are letters, use V = np.zeros(len(ordered_df)) or V = ordered_df.index - ordered_df.index for the .quiver direction vector.
• Plot the dataframe directly with pandas.DataFrame.plot and kind='scatter'.
• Tested in python 3.8.12, pandas 1.3.3, matplotlib 3.4.3

import pandas as pd
import numpy as np
import matplotlib.pyplot as plt

# Create a dataframe
np.random.seed(354)
value1=np.random.uniform(size=20)
value2=value1+np.random.uniform(size=20)/4
df = pd.DataFrame({'group':list(map(chr, range(65, 85))), 'value1':value1 , 'value2':value2 })
 
# Reorder it following the values of the first value and reset the index so the index values correspond to the y-axis tick locations
ordered_df = df.sort_values(by='value1').reset_index(drop=True)

# plot the dataframe
ax = ordered_df.plot(kind='scatter', x='value1', y='group', color='skyblue', alpha=1, figsize=(8, 6), label='value1')
ordered_df.plot(kind='scatter', x='value2', y='group', color='green', alpha=1, ax=ax, label='value2', xlabel='Value of the variables', ylabel='Group')

# plot the arrows
V = ordered_df.index - ordered_df.index  # the Y direction vector is 0 for each
ax.quiver(ordered_df.value1, ordered_df.group, (ordered_df.value2-ordered_df.value1), V, width=0.003, color='gray', scale_units='x', scale=1)

# Add title with position
ax.set_title("Comparison of the value 1 and the value 2", loc='left')

# Show the graph
plt.show()