getElementById("synthesis_formula");
    synthesis_formula.innerHTML = "a = " + X[3].toFixed(2) + ", b = " + X[2].toFixed(2) + ", c = " + X[1].toFixed(2) + ", d = " + X[0].toFixed(2);
    synthesis_formula.innerHTML += "<br>";
    synthesis_formula.innerHTML += "P(x) = " + X[3].toFixed(2) + "x<sup>3</sup> + " + X[2].toFixed(2) + "x<sup>2</sup> + " + X[1].toFixed(2) + "x + " + X[0].toFixed(2);

    // back
    synthesis_context.beginPath();
    for(var j = 1; j < w; ++j){
        x1 = client_to_x(j-1);
     