Writing a dissertation, thesis or paper for an academic journal and wondering how to write a figure legend or caption? I’m here to help! In this post we’ll cover everything you should include, hints and tips plus some examples to help you write the best figure legends.
A quick note on figure legends vs figure captions. Sometimes the figure caption is considered the title and the figure legend refers to the accompanying explanatory text. Both terms generally seem to be used relatively interchangeably. In any case, figure legend is the phrase which is much more widely searched for and I’ll therefore be using this phrase throughout the post.
What is a figure legend?
Before we dive into how to write a figure legend, let’s start by addressing what a figure legend actually is.
Essentially the figure legend is the text accompanying a figure and is typically displayed underneath it in papers and reports. As mentioned above, you may also hear it being referred to as the figure caption.
The purpose of a figure legend is to enable the reader to understand a figure without having to refer to the main text of your manuscript.
Therefore, you must include in the figure legend all of the crucial information, such as what technique was used, what the figure is showing and any necessary details for how to interpret it in regards to statistical analysis and scale bars.
This sounds like a lot of information to include, and it can be! Just make sure to not get carried away and begin discussing the figure. Your figure legend should simply enable the reader to understand what the figure is showing so that they can evaluate it for themselves.
Let’s now break down all of the different information you should include in your figure legend.
What to include in your figure legend
Generally the figure legend should include:
- A title
- The materials & methods involved with the presented figure
- Results, though this isn’t always applicable
- Any other miscellaneous details such as explaining abbreviations or image scale
Title
- Keep it brief and let it give a flavour of the whole figure.
- If you’re including multiple panels it is important you keep the title applicable to all panels. If this is awkward to do, it’s possible that your panels should be split into separate figures.
- Usually you’ll have some control over what style you want the title to take. You could choose to describe the technique you’ve used or the key outcome. Which way you write the title will depend upon the content and your own preferences.
Figure legend example titles:
“Test set performance of convolutional neural network (CNN) methods”
Figure 2, Weisman et al (Open access)
“Receiver operating curves for metrics of PH measurement”
Figure 3, Cassady et al
“The deep learning network architecture.”
Figure 1, Baskaran et al (Open access)
“The effect of the parameter n on the results of vessel enhancement”
Figure 3, Zhang et al (Open access)
“Methodology to extract contrast to noise (C/N) values in the micro-CT scans of osteochondral plugs.”
Figure 2, Clark et al (me!) (Open access)
Materials & Methods
Some potential things to include for the materials and methods section of your figure legend are:
- Which techniques did you use? What equipment was used? For example for microscopy name the technique as well as any key parameters such as objective lens magnification. Don’t go overboard describing all parameters, only the ones which relate to the presentation of the figure.
- What were the sample / treatment groups? Sample size should be made very clear so be sure to include how many samples were tested. For example “n = 9“
- If appropriate, outline statistical analyses carried out. This should include P-values. Additionally, if you’ve included confidence intervals, standard deviations, range or other metrics in error bars or brackets be sure to include details here.
If you’re including multiple panels in one figure, now is the time to describe any differences in materials or methods between the different panels. Typically you’ll label each panel alphabetically then describe them sequentially.
Figure legend example methods sections:
“Osteochondral explants were punched from the still macroscopically preserved looking knee condyle area and taken into culture. [A] Explants were subjected to treatment with IL-1β (10 ng/ml), triiodothyronine (T3; 10 nM) or warfarin (50 μM). [B] Explants received mechanical stresses at a strain of 65% for 10 minutes per day on four subsequent days. On day 13, cartilage and bone was separated, snap frozen and stored at -80C. [C] Schematic representation of the dynamic (cyclic) compression applied to osteochondral explants.”
Figure 1, Houtman et al (Open access)
“Maximum intensity projections (MIPs) of PET images with overlaying MIPs of the 3CNN output show true-positive findings (green), false-positive findings (red), and false-negative findings (blue).”
Figure 3, Weisman et al (Open access)
“(a) is the original image; (b) is the zoomed area marked in (a); (c), (d), and (e) is the results of vessel enhancement when the parameter n is 0.3, 0.5, 0.7, and 0.9 respectively.”
Figure 3, Zhang et al (Open access)
“Mean contrast to noise ratio (C/N) values (with 95% CIs) through the height of osteochondral plugs prepared with various methods and micro-CT scanned using the two Absorption Contrast (A.) and Propagation Phase (B.) protocols. Chondrocyte grayscale intensity (n 1⁄4 10) was plotted across 11 scan layers per sample as described in Fig. 2. The scans included a depth of approximately 2 mm of articular cartilage mid-way through its height. 100 micro-CT slices equates to approximately 200 mm. For comparison, data collected without cellular features is given as a control for signal variation in the surrounding matrix.”
Figure 4, Clark et al (me!) (Open access)
Results
A description of results is not always appropriate for a figure legend. Unless you’ve elected to write your title outlining the main result, reserve any mention of results for components which are:
- Noteworthy
- Anomalous
- Erroneous
Figure legend example results sections:
“The plate-like structure marked with red arrow in (c) is falsely enhanced when the n = 0.3.”
Figure 3, Zhang et al (Open access)
“A peak in the height of the bone, seen on the left-hand side of the defect (*), impeded loading of the cartilage and hybrid.”
Figure 3, Clark et al (me!) (Open access)
“On the contrary, all spatial distance based metrics discover the similarity and give (b) a higher score than (a). However, the metric most invariant to boundary error is the volumetric similarity, since it gives a perfect match in both cases”
Figure 6, Taha and Tanbury (Open access)
Make sure you’re not overtly diving into discussion of the figure. If you choose to include results in your figure legend just outline the key results and highlight noteworthy components, don’t begin a thorough discussion of why things are a certain way.
Miscellaneous details
Ensure that any other necessary details of what is displayed in the figure are described. I suggest going over your figure panels element by element and ensuring that it is clear what is being shown. This includes:
- Spelling out uncommon abbreviations
- Explaining what any different colours, lines or symbols are showing.
- If you’ve included scale bars without stating the actual values, which is sometimes preferred by journals, address what scale the bar is referring to!
Figure legend example details:
“The ground truth annotation is in green and the model prediction in red. Abbreviations: SVC = superior vena cava.”
Figure 4, Baskaran et al (Open access)
“Displayed values are mean and 95% CIs.”
Figure 7, Clark et al (me!) (Open access)
“The color intensity of each cell represents the strength of the correlation, where blue denotes direct correlation and red denotes inverse correlation”
Figure 3, Taha and Tanbury (Open access)
“Dots represent outliers, which are determined by points lying further than 1.5 times the interquartile range from the edge of the box.”
Figure 5, Weisman et al (Open access)
“The dashed line represents displacement of the compression platen.”
Figure 3, Clark et al (me!) (Open access)
Figure legends examples
Now that we’ve considered the separate elements to include in a figure legend, let’s tie it all together with some complete example figure legends:
Figure legend example 1
“Fig. 1. Renderings of the anatomy of the lungs. Image (a) shows a rendering of the lungs subdivided into the right upper (RU), right middle (RM), right lower (RL), left upper (LU), and left lower (LL) lobe. Image (b) shows a rendering of the vessels (red) and bronchi (blue) tree of the right lung. There are no major supply branches at the lobar boundaries (arrows).”
Lassen et al
Figure legend example 2
“Figure 3. Validation of localised strain against manually calculated values. Intra-Sample strain measurements calculated both using DVC software and manually calculated for one scaffold sample (Mech1), which underwent two levels of compression with in situ mechanical testing during micro-computed tomography (micro-CT) scanning (Table 1).”
Clark et al (me!) (Open access)
Figure legend example 3
“Fig. 1. U-net architecture (example for 32×32 pixels in the lowest resolution). Each blue box corresponds to a multi-channel feature map. The number of channels is denoted on top of the box. The x-y-size is provided at the lower left edge of the box. White boxes represent copied feature maps. The arrows denote the dierent operations.”
Ronneberger et al (Open access)
And a few longer examples:
Figure legend example 4
“Fig. 6. Micro-CT comparison with histology. (A.) Reconstructed micro-CT volume and light microscopy (LM) histological slices (Alcian Blue, Masson’s Trichrome; Picro-Sirius Red and Safranin O) of sample (e): 70% EtOH and PTA staining. The scale of the micro-CT and histology images is comparable. (B.) A high magnification region of interest between the two techniques is shown with symbols denoting corresponding features. Equivalent 500 x 500 μm areas were analysed between both methods (each n = 36) and mean values (with 95% CIs) are shown for cellular density (C.) and cellular roundness (D.). Measurements for lacunae encompass the chondrocytes and other cellular features.“
Clark et al (me!) (Open access)
Figure legend example 5
“Fig. 2 Testing the proposed tool against the ITK implementation using brain tumor segmentation. Comparison between the performance of the proposed evaluation tool and the ITK Library implementation in validating 240 brain tumor segmentations against the corresponding ground truth using the HD in (a) and the AVD in (b). The grid size (width x height x depth) is on the horizontal axis and the run time in seconds is on the vertical axis. The data points are sorted according to the total number of voxels, i.e. whd”
Taha and Tanbury (Open access)
My top tips for how to write a figure legend
- Keep it succinct but include all the necessary details as outlined above. Don’t worry too much about brevity in your first draft, just get the necessary details down. You can always make it more snappy and cull unnecessary details in later drafts.
- Make sure you mention all panels if you’ve included multiple panels in your figure.
- Check if the journal (or thesis) guidelines have any recommendations for writing a figure legend. Although uncommon, they may stipulate requirements for what must and mustn’t be included.
- Stay consistent with abbreviations, labelling and tense.
Also, this final tip isn’t a specifically for how to write a figure legend but is a great tip generally for inserting figures and legends to your draft. If you’re using Word, make sure you’re using the built-in caption function!
To do so:
- Insert your figure
- Right click on the figure and select Insert Caption then type out your figure legend. You can apply this also to Tables and Equations.
- Once you’ve written your figure legend, now when you’re referring to the figure in your text use Insert>Links>Cross-reference. Change the Reference Type to Figure and set Insert Reference To to “Only label and number”. Now you can easily reference your figures and add new ones without having to worry about manually updating the figure numbers!
Need help with other aspects of writing an academic paper? I’ve written a whole series of posts detailing the process and it’s also now available as a free eBook. Click the image below to see more.
I hope that this post has helped answer your questions about how to write a figure legend.
A reminder that I have a whole series of post detailing the various parts of writing a paper which are all available here. If you have any more questions relating to any aspect of PhDs or academia please let me know and I’ll write some content answering them.
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Sir am a student of msc zoology ,and i want to write a thesis for admission in foreign university,can you help me please?
Hi Yasmeen, thanks for sharing your comment. Beyond the content already on this website (start here if you’ve not already) I can jump on a quick 15 minute call with you if that would help? Otherwise, unless you’d like to book a session through this page, I’ll add this topic to my list of upcoming blog posts so that the content can help other future applicants too.