Possible error in code and feedback ; Machine learning for Robotics

I was working on the Machine learning for robotics course, unit 2, the very second program

import warnings
import matplotlib
import matplotlib.pyplot as plt
from matplotlib import cm
from mpl_toolkits.mplot3d import Axes3D
import numpy as np
import pandas as pd

# class definition

class LinearRegression(object):

    def __init__(self):

        self._m = 0
        self._b = 0
    def m(self):

        return self._m

    def b(self):

        return self._b

    def fit(self, X, y): # IMPLEMENTATION ABOVE EQUATIONS TO COMPUTE: m, b  

        X = np.array(X)
        y = np.array(y)
        X_ = X.mean()
        y_ = y.mean()
        num = ((X - X_)*(y - y_)).sum()
        den = ((X - X_)**2).sum()
        self._m = num/den
        self._b = y_ - self._m*X_

    def predict(self, x):

        x = np.array(x)
        return self._m*x + self._b

# Computation of MSE and regression (we use the same formulas as we defined earlier)

def MSE(ax, x, y, model):

    error = y - model.predict(x)
    MSE = (error**2).sum()/error.size
    ax.plot([x, x], [error*0, error])
    return MSE

def compute_regression(ax, x, y, model):

    error = y - model.predict(x)
    MSE = (error**2).sum()/error.size
    ax.scatter(x, y, label='distance')
    ax.plot([x, x], [y, model.predict(x)], ':')
    ax.plot(0, 0, ':', alpha=0.5, label='error')
    ax.plot([0, 100], model.predict([0, 100]), color='red', label='regression')
    ax.axis([0, 100, 0, 22])

# model is a object of class
model = LinearRegression()

# load dataset
data = pd.read_csv('/home/user/catkin_ws/src/machine_learning_course/dataset/test_brakes.csv')

# remove Unmaned column
data.drop(['Unnamed: 0'], axis=1, inplace=True)

# our data set
yA = (data.iloc[:,0].values).flatten()
x = (data.iloc[:,1].values).flatten()

plt.figure(figsize=(10, 8))
axA = plt.axes(xlim=(0, 100), ylim=(0, 22), autoscale_on=False)
model.fit(x, yA)
compute_regression(axA, x, yA, model)
plt.xlabel("% of max speed of axis 1", fontsize=16)
plt.ylabel("stop distance [deg]", fontsize=16)
plt.title("Linear regression", fontsize=18)


Question 1 In this we are using the function MSE even though we defined it, is it an error or did i miss something?

def MSE(ax, x, y, model):

    error = y - model.predict(x)
    MSE = (error**2).sum()/error.size
    ax.plot([x, x], [error*0, error])
    return MSE

**Quesion 2:**Can someone explain the following line?

    ax.plot([x, x], [error*0, error])

Most of the content regarding matlab plots was converted in the “Math Basics course” but I think the course would benefit if some more explanation was provided on the matlab functions since the Matlab functions used in the above code as not covered in the math basics course.