This should look familiar to those who are of sufficient age or interest to have worked with vacuum tubes, this is an example of "plate curves" for a 12A4 triode vacuum tube.

Using this set of curves, a designer can determine the best plate voltage, grid biasing and plate current for the intended circuit appoication. The horizontal axis represents plate voltage, the left hand side vertical axis represents plate current in milliamperes, and the negative numbers assigned to the curves represents the grid voltage. The maximum "plate dissipation" for this particular tube is 6.5 watts, which is the product of the plate voltage multiplied by the plate current.