How to fix the graphing line on math calculator with initial scale [Resolved]

Question

Im currently building an math graphing calculator in discord.js, the graphing work fine on 1.0x scale but when im changing the scale the graphing line start not right with the axes, I saw it to be move to bottom-left not to be the center of the graph

function draw(tree) {
    var i = 0;
    dx = 1 / global_scale;

    var y;
    variables.x = minX;

    var width = canvas.width;

    while (isNaN((y = eval(tree))) && i < width) {
        variables.x = minX + i * dx;
        i++;
    }

    let previousY = (y - minY) * global_scale;

    for (; i < width; i++) {
        variables.x = minX + i * dx;
        y = eval(tree);

        if (isNaN(y)) {
            console.log(`discontinuity at x = ${x}`);
            while ((y = eval(tree)) === NaN && i < width) {
                variables.x = minX + i * dx;
                i++;
            }
            previousY = (y - minY) * global_scale;
            continue;
        }

        y = (y - minY) * global_scale;

        ctx.beginPath();
        ctx.moveTo(i - 1, previousY);
        ctx.lineTo(i, y);
        ctx.lineWidth = 2;
        ctx.stroke();
        previousY = y;
    }
}```

[The graphing when i do x^2 with 0.01 initial scale][1]

[The graphing when i do sin(x) with 0.01 initial scale][2]


and i wanna it to be something like this :
[The graphing when i do x^2 with 1.0 initial scale][3]


  [1]: https://i.stack.imgur.com/WTTl6.png
  [2]: https://i.stack.imgur.com/8fH4n.png
  [3]: https://i.stack.imgur.com/hefWl.png