Trying to change a char variable in a 2D vector, which is also a class object. Keep getting "Thread 1: EXC_BAD_ACCESS (code=1, address=0xe)" error

Question

I'm trying to build 2 classes, one that creates a "function" object (Y = MX + B) and another that creates a Graph object, takes the function, and plots every Y value along the graph by inserting x values from -10 to 10 into the function. I have a loop which should take every Y value and place it along the graph, so long as it fits within the boundaries (the 2D vector is 21 elements tall and 21 elements wide). For some reason, when I try to actually replace the '-' char (that I have throughout the empty graph) with the '*' char, I get that error above.

Below is my Graph.cpp file.

// EDIT: Here is a MRE
#include <iostream>
#include <vector>
using namespace std;

class Graph
{
public:
    Graph();
    void displayGraph();
    Graph editGraph();
private:
    vector<vector<char>> yaxis;
    vector<char> line;
    vector<char> axis;
    
};

Graph::Graph()
{
    axis = {'_','_','_','_','_','_','_','_','_','_','|','_','_','_','_','_','_','_','_','_','_',};
    line = {'-','-','-','-','-','-','-','-','-','-','|','-','-','-','-','-','-','-','-','-','-'};
    yaxis = {line,line,line,line,line,line,line,line,line,line,axis,line,line,line,line,line,line,line,line,line,line};
}

void Graph::displayGraph()
{
    int i = 0;
    do
    {
        int z = 0;
        do
        {
            cout << yaxis[i][z];
            z++;
        }while(z < 20);
        cout << endl;
        i++;
    }while (i < yaxis.size());
}

Graph Graph::editGraph()
{
    Graph g1;
    int i = 0;
    int x = -10;
    int m1, b1;
    m1 = 2;
    b1 = 3;
    do
    {
        int yVal = (m1 * x) + b1;
        if((yVal + 10) >= 0 && (yVal + 10) <= 21)
        {
            g1.yaxis[(yVal+10)][i] = '*';
        }
        x++;
        i++;
    } while (i < 21);
    
    return g1;
}


int main()
{
    Graph g1;
    g1 = g1.editGraph();
    g1.displayGraph();
}

I tried placing breakpoints along the loop in the editGraph function, but for I just can't make sense of this specific error and what it means for my code. Is the problem the manner by which I'm inserting the new char? should I be using .at()? I've looked at other questions here with the same error, but they don't seem to be using vectors like I am.