A chart is simply a graphical representation of data, most commonly in graph form. Charts on websites are basically the same, but we can add interactivity to them.
Charts are great for tracking information on web administration interfaces such as billing or web-traffic tracking summaries. You can also use charts on website brochure pages as marketing tools (a great visual alternative to static images). Used in this way you could show statistics on the success of a product or growth in a market you operate in.
Web Charting Tools