Charts in lecture slides: which chart, how to label it
A chart on a lecture slide has one job: make a single comparison visible in a few seconds. That rules out most charts that come straight from a paper or a spreadsheet — they carry four comparisons at print size for a reader who can stop and study. The lecture version keeps one message, drops the rest, uses type large enough to read from the back row, and states the conclusion in the slide's title.
This guide covers the chart type, labelling, adapting a figure you did not make, and the check before class.
Pick by the comparison
| The comparison | The chart |
|---|---|
| Value across categories | horizontal bars, sorted by value |
| Change over time | line, with time on the x-axis |
| Part of a whole, two or three parts | stacked bar, or say the numbers out loud |
| Relationship between two variables | scatter, with the trend drawn if you will discuss it |
| Distribution | histogram, or a box plot if the audience knows the convention |
Two to avoid in a lecture: pie charts with more than three slices (nobody compares angles quickly), and dual-axis charts (they invite the comparison the axes do not support). The slide design guide applies to figures too — one idea per slide.
Label it for the back row
- The title states the finding, not the variables: "Throughput drops after 8 threads", not "Throughput vs threads".
- Axis labels with units, always. A number without a unit is a guess.
- Direct labels on the lines instead of a legend, when there are three or fewer series. The eye should not travel to a key and back.
- Type at least 18–20 px equivalent on a 16:9 slide; figures exported at print size are usually half that. The font size guide has the sizes that survive a lecture hall.
- Colour that still works when projected — many rooms wash out light colours, and some of your students do not distinguish red from green. Vary lightness or line style as well as hue; the accessible slides guide covers the rest.
Adapting a figure from a paper
Papers' figures are dense on purpose. Before it goes on a slide:
- Decide the one sentence the figure is there to support.
- Delete the panels that do not carry it. A four-panel figure usually becomes one panel, or two slides.
- Enlarge the type and thicken the lines. Redrawing from the underlying data is better when you have it.
- Keep the source on the slide — author, year, figure number. That is the same rule as citing sources in slides, and licences matter: see images and licensing.
If the figure is genuinely complex and the point is its complexity, show it whole, say so, and then show the simplified version. Building it up over two or three slides is the standard move and it works.
Building the figure
Whatever draws it — a plotting library, a spreadsheet, a drawing tool — export at 2× the slide size so it stays sharp on a projector, in PNG for plots with text, SVG if your pipeline keeps vectors. In Marp, an image is one line and sizing is a keyword:
*Throughput drops after 8 threads (Ahmed et al., 2024, fig. 3)*
w:900 is Marp's own sizing keyword; center only centres the figure if the theme has the one-line rule img[alt~="center"] { display: block; margin: 0 auto; } — see centring images and text.
Keep the script that generated the figure next to the image so next year's version is a re-run, not a redraw. For diagrams rather than data — architectures, state machines, flows — Mermaid in Marp draws them from text, which survives editing far better than a screenshot of a drawing tool.
The check before class
Stand at the back of the room, or shrink the slide to a quarter of your screen. If you cannot read the axis labels or tell the series apart, neither can the students in row twelve. It takes one minute and catches nearly every figure problem there is.
In Lecture Studio
Lecture Studio writes the deck and the notes from your sources; figures stay yours to make, and the Markdown it produces takes an image with one line. The app is free during the beta and needs an Oberik project key.
FAQ
What chart should I use on a slide?
Bars for values across categories, lines for change over time, scatter for a relationship. One comparison per chart.
How big should chart text be on a slide?
Large enough to read from the back — roughly 18–20 px equivalent on a 16:9 slide, which is usually twice what a paper figure uses.
Can I put a figure from a paper on my lecture slides?
Often, but it is a licensing question, not a technical one: check the publisher's terms, cite the source on the slide, and simplify the figure for the room.
Are pie charts bad for presentations?
With two or three slices they are fine; beyond that people cannot compare angles quickly. A sorted bar chart nearly always reads faster.