Period derivative vs. period for 204 of the 214 known nulling pulsars for which a period and period derivative measurement are available. These pulsars are classified into five categories, that is, those without a nulling fraction estimation (small black dots), those with a nulling fraction of less than or equal to 1% (light blue “x”), those with a nulling fraction larger than 1% but less than 30% (blue “+”), those with a nulling fraction larger than or equal to 30% (large red dots), and intermittent pulsars. The two sloping lines are the constant rates of loss of rotational kinetic energy Edot = 10³², 10³⁰ erg s⁻¹.