Построение графика функции на заданном интервале

double func1(double x)
{
  return 4*x - x*x;
}
//---------------------------------------------------------------------------
double func2(double x)
{
  return x*x*x - x*x - x - 1;
}
//---------------------------------------------------------------------------
void __fastcall TForm1::ButtonClick(TObject *Sender)
{
  Refresh();
 
  double x0 = -5, x1 = 5, step = 0.01, axis_shift = 50, r = 2, temp_x, temp_y[2];
  TColor color[3] = {clGray, clRed, clGreen};
  String text[2] = {"y=4*x-x^2", "y=x^3-x^2-x-1"};
 
  Canvas->Pen->Color = color[0];
  Canvas->MoveTo(0, ClientHeight/2 - axis_shift);
  Canvas->LineTo(ClientWidth, ClientHeight/2 - axis_shift);
  Canvas->MoveTo(ClientWidth/2, 0);
  Canvas->LineTo(ClientWidth/2, ClientHeight);
 
  Canvas->Pen->Width = 1.5 * r;
  Canvas->Brush->Color = Color;
  for (int i = 1; i <= 2; i++)
  {
    Canvas->Pen->Color = color[i];
    Canvas->MoveTo(0.5 * axis_shift, i * axis_shift / 2);
    Canvas->LineTo(1.5 * axis_shift, i * axis_shift / 2);
    Canvas->TextOutW(1.75 * axis_shift, i * axis_shift / 2 - 7, text[i-1]);
  }
 
  Canvas->Pen->Width = 1;
 
  for (double x = x0; x <= x1; x += step / 2)  // шаг уменьшен для лучшего отображения графиков
  {
    temp_x = (x - x0) * (ClientWidth - 2 * axis_shift) / (x1 - x0) + axis_shift,
    temp_y[0] = ClientHeight / 2 - func1(x) * (x1 - x0) - axis_shift,
    temp_y[1] = ClientHeight / 2 - func2(x) * (x1 - x0) - axis_shift;
 
    for (int i = 0; i <= 1; i++)
    {
      Canvas->Brush->Color = color[i+1];
      Canvas->Pen->Color = color[i+1];
      Canvas->Ellipse(temp_x - r, temp_y[i] - r, temp_x + r, temp_y[i] + r);
//    Canvas->Pixels[temp_x][temp_y[i]] = color[i+1];
    }
  }
}