MYP Physics: Drawing Graphs & Lines of Best Fit

Drawing graphs and lines of best fit means plotting your measured data points on labelled axes, then drawing a single straight line (or smooth curve) that passes as close as possible to all of them — not through every point. It's how MYP Physics turns raw numbers into a visible pattern you can use to find a gradient or intercept.

Real measurements always carry small errors, so joining dots zig-zags around random noise instead of showing the actual relationship. A line of best fit averages out that scatter and reveals the true trend the data is pointing to — exactly what you need to find a gradient or predict values inside (interpolate) or beyond (extrapolate) your data range.

A line of best fit is straight, used when the relationship between variables is linear — distance against time at constant speed, for example. A curve of best fit is a smooth, unbroken curve for non-linear data, such as period squared against pendulum length, or a radioactive decay graph. Both ignore anomalies and follow the general trend, not every wobble.

Plot every data point accurately, then look at the overall shape before your ruler touches the paper. Draw one straight line with roughly equal numbers of points above and below it, passing as close as possible to all of them — it doesn't need to touch a single point, and it should never zig-zag between them.

Put the independent variable — the one you changed — on the x-axis, and the dependent variable you measured on the y-axis, both labelled with quantity and unit, like Force / N. Choose a scale that spreads your data across at least half the grid in both directions; awkward scales like multiples of 3 make plotting and reading values far harder than they need to be.

Circle it — don't erase it. An anomalous point is a real result sitting well off the trend, often from a measurement slip rather than a genuine effect. Leave it plotted, exclude it from your line of best fit, and note in your evaluation that you identified and disregarded it, ideally with a reason.

Pick two points far apart on your line of best fit (not your data points), read their coordinates, and calculate gradient using $m=\frac{y_2-y_1}{x_2-x_1}$. The y-intercept is where your line crosses the y-axis — read it directly off the graph, including its unit, since that gradient usually represents a real quantity like speed or resistance.

Graphs and lines of best fit are assessed mainly under MYP Sciences Criterion C: Processing and evaluating, which asks you to present data using appropriate graphical forms, then interpret trends and evaluate the validity of your method. Poor axis labelling, wrong scales or a dot-to-dot line can cost marks even when your raw data collection under Criterion B was solid.

In years of marking, the same three errors show up constantly: joining dots instead of drawing a single best-fit line, forgetting units on axis labels, and squashing all the data into one corner of the grid with a poor scale choice. Each one is an easy fix once you know to check for it before submitting.

Graphs are judged against the Criterion C strands in the MYP Sciences guide — accurate, well-presented data (axes, scale, units) plus a sound line of best fit that supports a valid conclusion. At MYP 4-5, achievement levels run 0-8, and a graph with no clear line of best fit usually caps you in the lower bands regardless of correct data.

Most MYP schools allow — and often prefer — software like Excel, Google Sheets or Logger Pro for investigations, provided you still choose the trendline type yourself rather than accepting a default. The skill being assessed is understanding which line fits the data, not the pencil work, so check with your teacher which format your school expects.

The gradient-reading, uncertainty-spotting and line-fitting skills built in MYP 4-5 carry straight into DP Physics, where the Internal Assessment rewards exactly this under the Analysis criterion. According to the IB, the current DP Sciences guide, first examined in 2025, still expects graphical analysis with uncertainty and best-fit lines as core practical skills — so mastering it now saves real time later.

Repetition on real datasets works better than reading about theory — students need to plot, draw and calculate gradients from several small data sets before it becomes automatic. On RevisionPrep, MYP Physics Topical Worksheets and Revision Notes give practice data sets with model best-fit lines and worked gradient calculations, so students can check their own graph against a marked example.