var interval = 10; // DO NOT CHANGE THIS VALUE

// this is the equation.  You can change this
// to any equation, but just be sure to express 
// the equation in terms of y.  I _think_ this is
// slope intercept form, isn't it?
// Be aware, though, that javascript cannot handle y = -2x + 1
// you need to write it with * for multiply, / for divide
// so it would be var y = -2 * x + 1
var eq = function(x) { 
    //var y = x*x+1; // funky sample equation
    //var y = x*x*x/10+1; // funky sample equation
    var y = -2*x + 1; // CHANGE ME HERE

    return y;
};

// function to draw points of an equation.
var equation = function()
{
  // this loop will draw several dots
  // according to the equation above
    for(var x = -200; x <= 200; x++)
    {
        var y = eq(x); // use the function to calculate what y is
        stroke(255, 0, 0); // set colour of dots
        fill(255, 0, 0);   // set colour of fill
        ellipse( 200+x*interval, 200-(y)*interval, 5, 5 ); // draw dot
    }
};

// draw the origin
var drawOrigin = function()
{
    line(0,200,400,200); // draw x axis
    line(200,0,200,400); // draw y axis
    
    // loop to draw markers along x axis
    for(var x = 0; x < 400; x+= interval)
    {
        line(x,(200+interval/2),x,(200-interval/2));
    }
    
    // loop to draw markers along y axis
    for(var y = 0; y < 400; y+= interval)
    {
        line(200-interval/2,y,200+interval/2,y);   
    }
};

// fancy schmancy line matching the equation above.
// will not work correctly for anything other than 
// line in slope intercept form.
var equationLine = function()
{
    var x = -200;
    var x2 = 200;
    var y = eq(x);
    var y2 = eq(x2);
    
    stroke(0, 255, 0); // set line colour
    line(200+x*interval,200-(y)*interval,200+x2*interval,200-(y2)*interval);
};

// draw everything in a nice graph format
drawOrigin();
equation();
equationLine();