How To Write an Effective Figure Legend for a Laboratory Report

How To Write an Effective Figure Legend for a Laboratory Report

Figures are essential components of laboratory reports. They condense data, display patterns, highlight trends, and support key findings. However, a figure cannot communicate effectively on its own. To be meaningful, every figure must be accompanied by a precise, well-written figure legend. The legend acts as the reader’s guide, enabling them to interpret the visual information without needing to return repeatedly to the main text. In many laboratory courses and scientific journals, figure legends are treated as integral elements of scientific communication, and poorly written legends can undermine otherwise strong research.

This article explains how to construct clear, concise, and accurate figure legends. It addresses the decision between using captions and legends, outlines the essential components of a strong legend, discusses formatting and stylistic considerations, and offers guidance for maintaining consistency across multiple figures within one report. While exact requirements vary across disciplines, institutions, and publishers, the principles below represent best practices widely accepted in scientific writing.

1) Figure legends vs. figure captions: what is required?

Your first task is to determine whether your instructor, laboratory manual, or target journal requires figure legends, figure captions, or both. Although the terms are sometimes used interchangeably in casual contexts, they refer to different elements in formal scientific writing:

• Figure captions usually function as titles placed below a figure, briefly describing what the figure shows.
• Figure legends provide a fuller textual explanation, clarifying methods, conditions, symbols, abbreviations, and results.

Some reports require only a caption, placing the detailed explanatory material in the main text. Others require both a caption and a legend, with the caption serving as a succinct title and the legend acting as the figure’s “mini-methods-and-results” section. When both elements are required, the caption normally appears beneath the figure, while the legend may be incorporated inside the figure itself, often enclosed within a shaded or bordered box.

If only one element is required, it will almost always appear below the figure, formatted to match the width of the figure and usually in a slightly smaller font than the main text. Always follow the style specification provided, as journals differ significantly in their expectations.

2) The purpose of a figure legend

An effective figure legend serves three fundamental roles:

• Explanation: it clarifies what the figure shows, summarising the key information.
• Context: it provides sufficient experimental detail to allow interpretation without referring back to earlier sections.
• Definition: it defines all symbols, abbreviations, statistical markers, units, and visual patterns.

Readers should be able to understand the figure entirely on its own. A well-written legend is therefore a self-contained explanation—scientifically accurate, textually complete, and stylistically clear.

3) Structuring an effective figure legend

Although legends vary in length and complexity depending on the figure, most follow a predictable structure. A typical legend includes:

1. An opening descriptive statement
   This may describe the experiment, summarise the methodology, or state the primary result. It sets the stage for the rest of the legend.
2. A brief explanation of methods or conditions
   This is especially important if the figure displays experimental results without a separate Methods or Results section. Even when such sections exist, a condensed methodological summary helps readers interpret the figure.
3. Clarification of variables, treatments, groups, or experimental conditions
   Identify independent and dependent variables, sample sizes, control groups, and any relevant environmental or procedural details.
4. Definitions of symbols, colours, patterns, or abbreviations
   Any element that might be unclear must be defined. Legends often list these definitions separated by semicolons for clarity.
5. Explanation of statistical information
   Include confidence intervals, error bars, p-values, or levels of statistical significance when relevant.
6. Attribution of borrowed or adapted elements
   If the figure includes previously published material, credit the original source directly in the legend.

Depending on guidelines, this structure may be compressed or expanded. The key principle is simple: include everything a reader needs to interpret the figure accurately—but nothing unnecessary.

4) How much methodological detail should a figure legend include?

The appropriate level of detail in a legend depends on the overall structure of the report. In laboratory courses, students may produce reports where the figures replace the Methods or Results sections. In such cases, legends must include more comprehensive detail, such as:

• sample preparation methods,
• equipment or instruments used,
• temperature or reaction conditions,
• timing of observations,
• controls and treatments,
• data transformation or statistical tests applied.

If the report already contains detailed Methods and Results sections, the legend can be shorter, summarising only what readers need to interpret the visual information.

No matter the context, accuracy is essential. Legends must not contradict the main text, and they must align exactly with the figure in terminology, units, colours, markers, and abbreviations. Inconsistent terminology confuses readers and undermines your credibility.

5) Clarity, conciseness, and complete sentences

Most figure legends are written in complete sentences. However, the language must be concise. Long legends that read like paragraphs of a report can overwhelm readers. Short, clear sentences and semicolon-separated lists often work well.

Examples of concise elements:

• “Error bars represent ± 1 SD.”
• “Blue circles = treatment group; red squares = control group.”
• “Measurements were taken at 30 s intervals.”

The aim is not to reduce length at the expense of precision but to eliminate unnecessary words while preserving meaning.

6) Symbols, units, abbreviations, and terminology

Every symbol, abbreviation, and nonstandard term must be defined the first time it appears in a figure. This includes:

• Greek letters,
• statistical symbols (e.g., p, t, r, χ²),
• instrument-specific labels,
• colour keys, shading, line styles, and markers.

Consistency is crucial. Units used in the legend must match those in the figure, and both must match those in the main text. If “mg/mL” appears in the figure, the legend must not switch to “mg ml⁻¹.” Small inconsistencies distract readers and can lead to misinterpretation.

7) Acknowledging borrowed materials

If parts of a figure are adapted from or inspired by previously published work, provide full and accurate credit. Depending on the style guide, this might involve:

• “Adapted from Smith et al. (2019).”
• “Data reproduced with permission from…”
• “Graphic elements adapted from dataset…”

Using external material without attribution constitutes poor scholarly practice. Giving credit in the figure legend is the standard way to acknowledge visual borrowing.

8) Consistency across multiple figures

In a multi-figure report, legends must follow the same structure, formatting, terminology, and sequence. Readers should be able to move from Fig. 1 to Fig. 5 without having to decipher a new system of abbreviations or symbols.

Consistency improves the professionalism of your report. It also helps instructors and reviewers evaluate your figures more efficiently, as they do not need to re-learn your conventions for each individual legend.

9) Common mistakes to avoid

Students often make predictable errors when writing figure legends. These include:

• Insufficient detail: leaving readers unable to interpret the figure on its own.
• Too much detail: rewriting the entire Methods section in the legend.
• Inconsistent terminology: calling a variable “growth rate” in one figure and “increase per hour” in another.
• Undefined symbols: forgetting to explain markers or abbreviations.
• Incorrect placement: putting legends above figures when guidelines require them below.
• Inadequate proofreading: legends containing typographical or formatting errors.

A strong figure legend balances completeness with concision, accuracy with readability. Practise revising legends by asking whether each sentence is essential to a reader’s understanding of the figure.

10) Final thoughts

In scientific communication, clarity is a professional obligation. Figure legends are not supplementary flourishes but essential interpretive tools. A figure may be beautifully designed, but if its legend is vague or incomplete, its value disappears. A well-written legend enables a reader to understand your visual data without confusion or misinterpretation. By following guidelines carefully, applying consistent formatting, and writing with concision and precision, you can create legends that enhance the impact and credibility of your laboratory report.