The year is 1923. Imagine the patience needed to draw mathematics graphs like this by hand.
"In Fig. 82, the index is set for the two real roots of the biquadratic equation z⁴ + 1.33z³ + 1.6z² + 2z - 3.3 = 0."
From "The design of diagrams for engineering formulas & the theory of nomography," by L. Hewes & H. Seward, 1923, NY, McGraw-Hill.