Figures & presentations

How to make a good figure and how to give a talk

Lars Pastewka

Learning objectives

After this lecture you can

apply the eleven rules for better figures to your own plots
spot misleading axes, bad colour maps, and chartjunk – and fix them
structure a short scientific talk around a simple story
prepare the final colloquium presentation (including the scaling plot)

Why figures matter

Of the people who decide they like the title of your paper:

Share	… will
1%	actually read the paper
5%	read the abstract
10%	read the figure captions
100%	look at the figures

Your story should be in the figures.

Eleven rules for better figures

Condensed from Rougier, Droettboom, Bourne, Ten simple rules for better figures, PLOS Comput. Biol. 10, e1003833 (2014). They are mandatory for the final colloquium.

Rule 1 – Read the labels

Every axis has a label; every label is legible at the back of the lecture hall.

If the reader has to squint, the figure is broken.

Rule 2 – Number panels, top-left

Multi-panel figures get (a), (b), (c) … in the top left of each panel.

Caption and talk refer to panels by letter – don’t make the reader hunt.

Rule 3 – Use space wisely

No wasted white space, oversized markers, or decoration – and no four ideas crammed into one figure.

Give the important content the space it deserves.

Rule 4 – Use identical scales

Panels meant to be compared get the same axis ranges and tick spacing.

Different scales hide the very effect you want to show.

Rule 5 – Points are data, lines are interpolation

Points = measurements; lines = a model, a fit, or linear interpolation.

Don’t draw a smooth curve through measurements unless it is a fit you can defend.

Rule 6 – Captions trump legends

The caption explains the figure; the legend only identifies the curves.

A skimming reader must understand the figure from the caption alone.

Rule 7 – Don’t mislead the reader

Same data – the truncated axis turns a 2% difference into a dramatic visual gap.

Rule 8 – Don’t trust the defaults

Same data – fix fonts, line widths, ticks, and label curves directly.

Rule 9 – Think about colour maps

Jet is not perceptually uniform and dies in greyscale – use viridis/cividis (or a diverging map around a meaningful zero).

Rule 10 – Avoid chartjunk

Backgrounds, heavy grids, shadows, legend frames: ink without information.

Rule 11 – Adapt the figure to the audience

A paper figure is dense; a slide figure is sparse.

Re-make figures for talks: bigger fonts, fewer curves, larger markers.

Message trumps beauty

A polished but unclear figure is worse than a rough but clear one.

Decide on the message first; let the visual decisions follow.

Units convention

Specific units in parentheses, ideally with the quantity name:

✓ σ (GPa) / ✓ Shear stress σ (GPa)
(ok) σ / GPa, σ in GPa (DIN style)
✗ σ [GPa] – never

How to give a talk

There is only one rule

Tell a (simple) story.

This means you have to know what your story is before you start making slides.

Corollary: anything that does not help the story has to be cut – less is more. Slides need to flow logically.

One message per slide

Your story flows through the slide messages. A message is a sentence, e.g. “Kokkos is slower than CUDA”.

One message per slide
Support that message with one (at most two) display items
Support that display item with conclusive, logical data

As little text as possible

While someone reads your slide, they are not listening to you.

Use bullet phrases, not paragraphs
Minimum 18 pt for body text, 14 pt for references
If you find yourself writing full sentences, you’ve gone too far

Preparation

Practice

Often – you will be nervous
More often – if you are presenting in a group

Learn by heart

The first three sentences (so you can start under stress)
Not the rest – it should sound like a conversation, not a recital

Polish your slides

Nice figures, consistent fonts, no overlapping logos / text

Dos

Face the audience – don’t read the slide off the screen
Explain your graphs one element at a time: “The x-axis shows … the y-axis shows … this curve is …”
Acknowledge external sources – citation on the same slide as the figure, not on a separate references slide

Don’ts

Don’t run over time. Minimum: one minute per slide.
Don’t excuse yourself (“This is the first time I am giving this talk, it will be terrible.”)
Don’t lie – stretching data to fit the hypothesis is a bigger sin than getting a null result
Don’t waste a slide on an outline / table of contents – the audience will have forgotten in 30 seconds
Don’t put all references on one slide at the end – references belong next to the figure or claim they support

Final colloquium

Structure of the talk

Section	Content
Introduction	What did you do? Which programming model?
Methods	How does your model work? How does it differ from what we did in the lecture? Advantages and pitfalls.
Results	Show the code works (a video of the lid-driven cavity is great). Performance: in million lattice updates per second.
Discussion	What did you learn? What would you do differently next time?

10–15 minutes talk + 15–20 minutes discussion. All group members present.

What a scaling plot should look like

Figure 1: Strong / weak scaling plot from Greiner & Pastewka, Proceedings of the bwHPC Symposium (2019). The \(x\)-axis is number of MPI processes; the \(y\)-axis is million lattice updates per second; the dashed lines indicate ideal scaling.

Summary

The three rules people break most:

Truncated axes that inflate small effects (Rule 7) – start bars at zero
Jet/rainbow colour maps (Rule 9) – use viridis or a diverging map
Default styling and chartjunk (Rules 8, 10) – tweak fonts, strip decoration

And for the talk: one simple story, one message per slide, as little text as possible.

Tools

Figures (free): Jupyter Lab, matplotlib, Inkscape, Gimp, IPE, Blender, ColorBrewer

Figures (commercial): Origin, DataGraph, Adobe Illustrator

Slides (open source): LibreOffice Impress, Quarto, Beamer (LaTeX), Jupyter + RISE, Marimo

Slides (commercial/free): PowerPoint, Keynote, SoftMaker Office, OnlyOffice, WPS Office

Reading

Figure rules