How to change the opacity of a plot using Matplotlib

import matplotlib.pyplot as plt
import pandas as pd

# We create our dataframe
df = pd.DataFrame(index=range(0,10), data={"col1" : range(0,10)})

fig, axes = plt.subplots(1,1, figsize=(8,6))

# We do a line plot on the axes
axes.plot(df.index,
          df["col1"],
          alpha=0.5) # see the alpha

# Fixing the layout to fit the size
fig.tight_layout()

# Showing the plot
plt.show()