Science

An inverse (or negative) relationship — as one variable increases, the other decreases. The opposite is a direct (positive) relationship.

A direct (or positive) relationship — both variables increase together. The line or curve slopes upward to the right.

The axis labels and their units (and the title/legend). Knowing what each axis measures and in what units prevents misreading values and reversing the relationship.

The independent variable goes on the x-axis (horizontal); the dependent variable goes on the y-axis (vertical). The dependent variable's value depends on the independent one.

To estimate a value that falls BETWEEN two measured data points — reading inside the range of the data you already have.

To estimate a value that falls BEYOND the range of the measured data — extending the trend past the last data point. It is less reliable than interpolation because the trend may not continue.