Charttoolbox is a free web based tool that generates heatmaps from tabular data. Its primary function is to transform data from CSV, Excel, or directly pasted spreadsheet cells into customizable visual heatmaps, including Pearson correlation matrices. The application runs entirely within the browser, meaning files are read and processed locally without being uploaded to any server. This design ensures that file names, contents, and values remain private and are not stored, inspected, or tracked. The tool supports file uploads up to 10 megabytes and 20,000 cells, accepting formats such as CSV, TSV, XLSX, and XLS. Users can also paste data directly from a clipboard or start with a built in sample dataset. Charttoolbox offers a range of customization options, including sequential, diverging, and color blind friendly palettes with adjustable limits and midpoints. Labels and scales can be refined with live previews. Output can be exported as a high resolution PNG for reports or as an editable SVG for publications and design software. Typical use cases include business reporting, where users compare performance across teams, regions, products, or time periods; academic research, which requires clean correlation matrices for papers and presentations; and exploratory data analysis, where categorical data can be pivoted and patterns or outliers inspected without writing chart code. The workflow involves uploading or pasting data, letting the tool automatically recognize the structure, customizing the visual design, and then downloading the resulting graphic. No account or sign up is required, and no server side file processing occurs.