Drawing horizontal mean/average line across a scatterplot

Question

im trying to draw a straight horizontal line across all the mean of data points of the blue and red color.

Right now, it looks like this

and this is my code for the above picture

fig=plt.figure(figsize=(20,10), dpi=100)
ax=fig.add_axes([0,0,1,1])
ax.scatter(x='Speed', y='Port ME Homemade Mass Flow Rate (kg/s)', data=df1before, color='r', marker='1')
ax.scatter(x='Speed', y='Port ME Homemade Mass Flow Rate (kg/s)', data=df1after, color='b', marker='x')
ax.set_xlabel('Speed(Knot)')
ax.set_ylabel('Port ME Homemade Mass Flow Rate (kg/s)')
#plt.plot(x=df3[len('Speed')], y=df3['Port ME Homemade Mass Flow Rate (kg/s)'].mean())
plt.show()

The comment is what I have tried.. The desired output would be:

It would also be great if each line could show the value of position Y that it's at.

Thanks!

Answer 1

ax.axhline draws a horizontal line at a given height, default going over the full width of the plot.

ax.text puts a text at a given position. Using the y-axis transform, the x-coordinate can be given as a position relative to the axes, and the y-coordinate as a data value.

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

y_name = 'Port ME Homemade Mass Flow Rate (kg/s)'
df1before = pd.DataFrame({'Speed': np.random.uniform(.1, 1, 100) ** 3 * 4,
                          y_name: np.random.normal(.053, .01, 100)})
df1after = pd.DataFrame({'Speed': np.random.uniform(.1, 1, 100) ** 3 * 4,
                         y_name: np.random.normal(.057, .01, 100)})

fig, ax = plt.subplots(figsize=(20, 10), dpi=100)
ax.scatter(x='Speed', y=y_name, data=df1before, color='r', marker='1', label='Before')
ax.scatter(x='Speed', y=y_name, data=df1after, color='b', marker='x', label='After')
ax.set_xlabel('Speed(Knot)')
ax.set_ylabel('Port ME Homemade Mass Flow Rate (kg/s)')
mean_before = df1before[y_name].mean()
ax.axhline(df1before[y_name].mean(), color='r', ls='--')
ax.text(1, mean_before, f'mean: {mean_before:.4f}\n',
        ha='right', va='center', color='r', transform=ax.get_yaxis_transform())
mean_after = df1after[y_name].mean()
ax.axhline(df1after[y_name].mean(), color='b', ls='--')
ax.text(1, mean_after, f'mean: {mean_after:.4f}\n',
        ha='right', va='center', color='b', transform=ax.get_yaxis_transform())
ax.legend()
plt.tight_layout()
plt.show()