Another colour missing in matplotlib

Question

When i try plot the following data in python, i do not see the green color portion in my graph. Please find it below. Meanwhile, please be noted that I use python 2.7.4.

import pandas as pd
import numpy as np
import matplotlib.pyplot as plt
plt.style.use('ggplot')
%matplotlib inline

range = pd.date_range('2015-01-01', '2015-12-31', freq='15min')
df = pd.DataFrame(index = range)
df

# Average speed in miles per hour
df['speed'] = np.random.randint(low=0, high=60, size=len(df.index))
# Distance in miles (speed * 0.5 hours)
df['distance'] = df['speed'] * 0.25 
# Cumulative distance travelled
df['cumulative_distance'] = df.distance.cumsum()

df.head()

fig, ax1 = plt.subplots()

ax2 = ax1.twinx()
ax1.plot(df.index, df['speed'], 'g-')
ax2.plot(df.index, df['distance'], 'b-')

ax1.set_xlabel('Date')
ax1.set_ylabel('Speed', color='g')
ax2.set_ylabel('Distance', color='b')

plt.show()
plt.rcParams['figure.figsize'] = 12,5

Answer 1

Speed and distance are two parameters which are directly proportional to each other. If you normalize speed/distance sets, you get exactly the same graph. As you draw your drafts with alpha=1 (opaque), then the only color you see is the last one drawn (blue). If you use alpha <> 1:

fig, ax1 = plt.subplots()

ax2 = ax1.twinx()
ax1.plot(df.index, df['speed'], 'g-', alpha=0.5)
ax2.plot(df.index, df['distance'], 'b-', alpha=0.1)

ax1.set_xlabel('Date')
ax2.set_ylabel('Distance', color='b')

ax1.set_ylabel('Speed', color='g')

plt.show()
plt.rcParams['figure.figsize'] = 12,5

you see the green color (in fact a mixture of green and blue):