[USER (data scientist)]: Awesome! Now, I'd like to visualize these approval rates using bar charts. Please generate and display a set of bar charts on a single figure, showing the approval rates by credit history, employment status, for the middle-age group (ages 40-59), and by credit amount range in the 'credit_customers' dataset.
My template of code snippet is:

---BEGIN CODE TEMPLATE---

import pandas as pd 
import matplotlib.pyplot as plt 
 
# Load the dataset 
credit_customers = pd.read_csv("credit_customers.csv") 

fig, axes = plt.subplots(2, 2, figsize=(12, 8)) 
 
# YOUR SOLUTION BEGIN:
<code1>
[COMPLETE YOUR CODE] 
</code1>
# YOUR SOLUTION END

plt.tight_layout() 
plt.savefig('ref_result/bar_chart.png')
plt.show()

---END CODE TEMPLATE---

[YOU (AI assistant)]: Sounds good! Let's create bar charts for each category using matplotlib. Here's the code for that:
'''
import pandas as pd 
import matplotlib.pyplot as plt 
 
# Load the dataset 
credit_customers = pd.read_csv("credit_customers.csv") 

fig, axes = plt.subplots(2, 2, figsize=(12, 8)) 
 
# YOUR SOLUTION BEGIN:
