A good scale makes a graph easy to plot and easy to read. Two rules guide the choice. First, the scale should be simple and even, such as one large square representing 1, 2, or 5 units, never an awkward value like 3 or 7, because odd scales make plotting slow and error-prone.
Second, the plotted points should occupy more than half of the graph grid in both directions. A graph squeezed into one corner wastes precision and makes the gradient harder to read accurately.
To set the scale, look at your largest data value on each axis and choose a scale that lets that value fit while filling the space. The axes do not always have to start at zero, though starting at zero is needed when you intend to read an intercept.
A sensible scale is quietly worth marks because every later step, plotting, the best-fit line, and the gradient, depends on it.
What examiners award
Marks are awarded for a simple even scale and for plotted points that fill more than half the grid in both directions, with the axes clearly numbered.
Common mark losses
In short
Choose a simple, even scale that spreads the data across more than half the grid. Awkward scales cause plotting errors and waste graph paper.
A routine that works
Practise the mechanics until they are automatic: label both axes with units, choose a scale that fills more than half the grid, plot points with a sharp pencil, draw a single best-fit line, and take the gradient from a large triangle with its coordinates and unit shown. One clean example beats re-reading the theory.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)