Python: Is it possible to change line color in a plot if exceeds a specific range?

Question

Is it possible to change the line color in a plot when values exceeds a certain y value? Example:

import numpy as np
import matplotlib.pyplot as plt
a = np.array([1,2,17,20,16,3,5,4])
plt.plt(a)

This one gives the following:

I want to visualise the values that exceeds y=15. Something like the following figure:

Or something like this(with cycle linestyle)::

Is it possible?

Answer 1

Basically @RaJa provides the solution, but I think that you can do the same without loading an additional package (pandas), by using masked arrays in numpy:

import numpy as np
import matplotlib.pyplot as plt

a = np.array([1,2,17,20,16,3,5,4])

# use a masked array to suppress the values that are too low
a_masked = np.ma.masked_less_equal(a, 15)

# plot the full line
plt.plot(a, 'k')

# plot only the large values
plt.plot(a_masked, 'r', linewidth=2)

# add the threshold value (optional)
plt.axhline(15, color='k', linestyle='--')
plt.show()

Result:

Answer 2

I don't know wether there is a built-in function in matplolib. But you could convert your numpy array into a pandas series and then use the plot function in combination with boolean selection/masking.

import numpy as np
import pandas as pd

a = np.array([1,2,17,20,16,3,5,4])
aPandas = pd.Series(a)
aPandas.plot()
aPandas[aPandas > 15].plot(color = 'red')