Overlapping Bar Charts

This article demonstrates how to create overlapping bar charts using HTML, CSS, and JavaScript, focusing on accessibility best practices. The author uses semantically rich HTML (description lists) to structure the chart, making it more accessible to screen readers. Flexbox is leveraged for efficient layout and positioning of chart elements. JavaScript dynamically sets bar heights based on data attributes, ensuring accurate visual representation. The article highlights the importance of color contrast for accessibility and provides code to improve keyboard navigation (using tabindex) and screen reader announcements (aria-label, aria-hidden). The author also discusses the use of a legend to clarify the chart's data representation and offers a visually hidden technique to prevent redundant screen reader announcements. The overall approach prioritizes both visual appeal and accessibility, making the chart usable for a wider audience.

Creating visually appealing and accessible data visualizations is crucial for effective communication. This tutorial details the construction of an overlapping bar chart, a powerful tool for comparing datasets. The key is leveraging HTML's semantic capabilities, CSS's layout power (specifically Flexbox), and JavaScript for dynamic element manipulation.

The foundation is a well-structured HTML utilizing <dl></dl> (description list) elements, a more semantic choice than unordered lists, providing better context for screen readers. This structure neatly separates the y-axis labels, x-axis labels, and the data bars themselves. Each bar's height is determined by a data-percentage attribute, dynamically adjusted with JavaScript.

CSS styling uses Flexbox for efficient layout. The .chart container is a flexible row, while the y-axis labels are arranged as a column. The bars themselves are also arranged flexibly, ensuring even distribution of space. Overlapping is achieved by strategically positioning bars and using z-index to control stacking order.

Accessibility is a core focus. The tutorial emphasizes sufficient color contrast to meet WCAG AA standards, providing specific color combinations and contrast ratios. JavaScript adds tabindex="0" to each bar, enabling keyboard navigation. Crucially, aria-label attributes clearly identify each bar's purpose (e.g., "Estimate," "Actual") for screen readers, while aria-hidden is used to prevent unnecessary announcements of non-essential elements. A visually hidden technique is employed to hide redundant numerical data while still making it accessible to screen readers.

The article concludes with a complete, accessible, and visually engaging overlapping bar chart, showcasing a best-practice approach to data visualization development. The author encourages alternative approaches and further discussion on accessibility best practices.

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