# import necessary libraries
from collections import Counter

# define the string
string = 'data science and analysis'

# define Counter object which counts chars
char_counts = Counter(string.replace(' ', ''))

# find the least frequent character
least_frequent = char_counts.most_common()[-1][0]

# replace missing values with the least frequent char
results = string.replace(' ', least_frequent)

# print the results
print(results)