A best-fit line summarises the trend of your data with a single straight line. The aim is balance: the line should follow the general direction of the points, with roughly as many points above it as below, and the scatter kept small on both sides.
Use a sharp pencil and a ruler to draw one thin, continuous line. Do not join the points dot to dot, and do not draw several trial lines. A single clean line is what earns the mark.
Do not force the line through the origin unless the data genuinely supports it. If the points trend towards a non-zero intercept, the best-fit line should reflect that honestly.
An obvious outlier that sits far from the trend can be treated as anomalous and ignored when positioning the line, provided the rest of the data forms a clear pattern. A well-judged best-fit line makes the gradient and intercept reliable.
What examiners award
Marks are awarded for a single thin straight line following the trend, with points balanced above and below and any clear anomaly ignored.
Common mark losses
In short
A best-fit line passes through the trend of the points with roughly equal numbers above and below. Draw one thin straight line and do not force it through the origin.
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)