We have implemented ECplot, an online tool for plotting charts from large datasets. This tool supports a variety of chart types commonly used in bioinformatics publications. In our benchmarking, it was able to create a Box-and-Whisker plot with about 67 000 data points and 8 MB total file size within several seconds. The design of the tool makes common formatting operations easy to perform. It also allows more complex operations to be achieved by advanced XML (Extensible Markup Language) and programming options. Data and formatting styles are stored in separate files, such that style templates can be made and applied to new datasets. The text-based file formats based on XML facilitate efficient manipulation of formatting styles for a large number of data series. These file formats also provide a means to reproduce published figures from raw data, which complement parallel efforts in making the data and software involved in published analysis results accessible. We demonstrate this idea by using ECplot to replicate some complex figures from a previous publication.