1 Introduction

We discussed the importance of visual representation in academic texts and pointed out that in the digital age, people are increasingly relying on charts to understand scientific data. Chart titles and legends are crucial for interpreting visual information and providing coherent information. With the globalization of academic knowledge, English has become the main language of communication, and Chinese manuscripts often need to provide chart titles in both Chinese and English, which poses a challenge for authors and editors. Existing research mostly focuses on the synchronic analysis of chart titles, while this article will adopt a diachronic perspective to study the evolution of chart titles through a self built corpus of marine science journal paper chart titles. The study will consider subgenres of chart titles and headers, and conduct cross genre analysis over time using 1982, 2002, and 2023 as time nodes.

2 Research Design

This chapter focuses on the field of marine science and explores the historical evolution of chart titles since the 1840s. The study selected three top ranked SCI marine science journals and randomly selected empirical papers from 1982, 2002, and 2023 to analyze the changes in chart titles in terms of length, intertextuality, lexical grammatical features, and syntactic complexity. Research has found that the length of graphic titles is generally longer than that of table titles, and the dispersion of graphic titles increases over time, indicating that their communicative purposes and content are more diverse and complex. The study also conducted manual extraction and text cleaning, using professional tools to statistically analyze keywords, standardized frequency differences, and syntactic complexity indicators to reveal the evolution trend of chart titles over time.

3 Results and Analysis

The study on the historical evolution of chart titles in academic journals of marine science reveals the evolution of the core elements of chart titles. In the early days, the structure of chart titles was not standardized and there was a cross page phenomenon, but the standardization was relatively fixed. There are many intertextual contents such as data sources, other charts, and references in the chart titles, reflecting the complexity of academic discourse. The use of keywords, articles, and prepositions reflects the pursuit of disciplinary characteristics and information conciseness. In marine science corpus, keywords such as lines, colors, etc. appear in the captions and headers of the illustrations, and the diversity of the corpus will increase in 2023. Personal name references are common in chart titles and headers, while paraphrasing verbs such as indicate and other highly relevant subject knowledge verbs appear in the form of - ed or - ing, reflecting the effective means of concise expression in chart titles. The use of indefinite articles in the header subgenre is lower than that in the subject text and decreases over time, while the frequency of definite article "the" is higher in the caption subgenre. There are differences in the use of prepositions in the header and caption, where the header uses more adjectives and adverbs instead of prepositional phrases, while the caption takes on more complex expressions. The hierarchical structure of prepositions and the complexity of noun phrases reflect typical characteristics of academic discourse. In the titles of marine science charts, the embedding level of prepositional phrases as postpositive modifiers is mainly one layer, but over time, the complexity of the header and graphic titles changes. The research suggests that in order to maintain the accessibility of readers' information and avoid increasing reading difficulty, the hierarchy of preposition usage should be controlled at levels 1-3.

4 Conclusion

Charts play a role as visual discourse in scientific writing and are crucial for the creation and dissemination of academic knowledge. The complexity of chart titles and headers in the field of marine science is high. Over time, the number of charts increases and the richness of information improves, reflecting the multimodal characteristics of academic discourse. The core elements of chart titles are intertextuality, and information conciseness and cognitive load threshold are reflected in the use of articles, prepositions, and noun phrases. The use of indefinite articles has decreased over time, while the use of definite articles has declined, reflecting the objective requirements of academic writing. The nested hierarchy of prepositions increases over time, but there is a threshold. The high priority of the chart allows it to attract readers' attention, but it needs to maintain high readability to facilitate understanding. Research indicates that the corpus size is limited, and further development is needed for the diachronic study of chart titles. The creation and evolution of chart titles have teaching value for academic English education, and educators should pay attention to the subgenre literacy challenges of beginners. Academic English professionals can enhance their multimodal discourse skills through corpus and digital humanities.