day                1     2      3     4      5     6

sales_dept1        0     0      0     3      3     3
sales_dept2        0     0      0     6      6     6
sales_dept3        0     0      0     1      1     1

dept1 slope = 1
dept2 slope = 0.5
dept3 slope = 3

from sklearn.linear_model import LinearRegression
regressor = LinearRegression()

X = [[0], [0], [0], [3], [3], [3]]
y = [1, 2, 3, 4, 5, 6]
regressor.fit(X, y)
print(regressor.coef_)