There are several lines which have been drawn on the graph. All of these lines are parallel to either the horizontal or vertical axis. To find the plate resistance there are two lines one horizontal and one vertical making up the sides of a right triangle. A portion of one of the grid voltage lines makes up the hypotenuse. These lines are drawn in red and labeled r sub p. The first line goes from 180, 2.33 to 220, 2.33. This line spans 40 volts horizontally. The attached line at right angles goes from 220, 2.33 to 220, 3.25. The hypotenuse is a part of the minus 1 volt grid voltage line. Over to the right and on the minus 4 volt line is 380, 2.12 to 420, 2.12, and another from 420, 2.12 to 420, 2.91. This will give a slightly different value of r sub p as explained in the regular text.
There are green lines for amplification factor. They are drawn horizontally along current lines. The first one runs along the 2.8 mA current line from the 0 grid voltage line to minus 2. The plate voltage values are 131 and 231. To the right is another line that runs along the 2.5 mA line from grid voltage lines minus 3 to minus 4. The voltages are 328 and 400.
There are magenta lines for transconductance that are vertical. The first one runs along the 200 volt plate voltage line from grid voltage of 0 to minus 1. The current values are, 2.8 and 3.61. The other runs along the 380 bolt line from minus 3 to minus 4. The current values are 2.12 and 3.7 mA. End verbal description.