Read the axes before the line
Check which quantity belongs on each axis, its units, and the size of each interval. A point (x, y) describes the horizontal coordinate first and the vertical coordinate second.
Connect a rule to the plotted values
A point on the graph of an equation satisfies that equation. For a straight line, slope is vertical change divided by horizontal change; its meaning depends on the quantities and units.
A mistake to avoid
A graph can look steeper after changing the axis scales without changing the underlying rate. Calculate the numerical changes instead of comparing the drawn angle alone.
Worked example
Read y = 2x + 1
When x = 0, y = 1; when x = 2, y = 5. The points (0, 1) and (2, 5) lie on the line. Its slope is (5 − 1)/(2 − 0) = 2, and its vertical intercept is 1.