How to take draw an average line for a scatter / a plot in MatPlotLib?

Question

My data is the following:

x = [3,4,5,6,7,8,9,9]
y = [6,5,4,3,2,1,1,2]

And I can obtain the following two graphs.

and

However, what I want is this (an average of all the points along the way):

Is it possible in matplotlib? Or do I have to change the list manually and somehow create:

x = [3,4,5,6,7,8,9]
y = [6,5,4,3,2,1,1.5]

RELEVANT CODE

ax.plot(x, y, 'o-', label='curPerform')
x1,x2,y1,y2 = ax.axis()
x1 = min(x) - 1 
x2 = max(x) + 1
ax.axis((x1,x2,(y1-1),(y2+1)))

Answer 1

I think this could be done most simply by doing y_mean = [np.mean(y) for i in x]

Example:

import matplotlib.pyplot as plt
import random
import numpy as np


# Create some random data
x = np.arange(0,10,1)
y = np.zeros_like(x)    
y = [random.random()*5 for i in x]

# Calculate the simple average of the data
y_mean = [np.mean(y)]*len(x)

fig,ax = plt.subplots()

# Plot the data
data_line = ax.plot(x,y, label='Data', marker='o')

# Plot the average line
mean_line = ax.plot(x,y_mean, label='Mean', linestyle='--')

# Make a legend
legend = ax.legend(loc='upper right')

plt.show()

Resulting figure:

Answer 2

Yes, you must do the calculation yourself. plot plots the data you give it. If you want to plot some other data, you need to calculate that data yourself and then plot that instead.

Edit: A quick way to do the calculation:

>>> x, y = zip(*sorted((xVal, np.mean([yVal for a, yVal in zip(x, y) if xVal==a])) for xVal in set(x)))
>>> x
(3, 4, 5, 6, 7, 8, 9)
>>> y
(6.0, 5.0, 4.0, 3.0, 2.0, 1.0, 1.5)