PNG by a long way. JPEG is designed for compressing photographs where a small loss in quality is usually acceptable (and not very noticeable). For chart images, where there are usually sharp distinctions between areas of color, the artifacts introduced by the JPEG format are very noticeable. Don't use JPEG unless you absolutely have to. The PNG format is "lossless", so chart images are always reproduced perfectly.

Source: JFreeChart FAQ

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