I'm making a game in android and I need plot a function, my algorithm is this:

    @Override
    protected void onDraw(Canvas canvas) {
        float e = 0.5f; 
        //from -x axis to +x evaluate f(x) 
        for (float x = -z(canvas.getWidth()); x < z(canvas.getWidth()); x+=e) {
                 float x1,y1;
                 x1 = x;
                 y1 = f(x);
                 canvas.drawPoint((canvas.getWidth()/2)+x1, (canvas.getHeight()/2)-y1, paintWhite);
        }   
        super.onDraw(canvas);
    }

This is how it works. If my function is, for example f(x)=x^2, then z(x) is sqrt(x). I evaluate each point between -z(x) to z(x) and then I draw them. As you can see I use the half of the size of the screen to put the function in the middle of the screen. The problem isn't that the code isn't working, actually plots the function. But if the screen is of 320*480 then this function will be really tiny like in the image below. My question is: how can I change this algorithm to scale the function?. BTW what I'm really wanting to do is trace a route to later display an animated sprite, so actually scale this image doesnt gonna help me. I need change the algorithm in order to draw the same interval but in a larger space. Any tip helps, thanks!

Current working result

Desired result

UPDATE: I will try explain more in detail what the problem is. Given this interval [-15...15] (-z(x) to z(x)) I need divide this points in a bigger interval [-320...320] (-x to x). For example, when you use some plotting software like this one. Although the div where is contain the function has 640 px of width, you dont see that the interval is from -320 to 320, you see that the interval is from -6 to 6. How can I achieve this?