plot2d ([atan(x), erf(x), tanh(x)], [x, -5, 5], [y, -1.5, 2],
  [pdf_file, "./introduction2.pdf"]);
plot2d ([atan(x), erf(x), tanh(x)], [x, -5, 5], [y, -1.5, 2],
  [png_file, "./introduction2.png"]);
