SetFileFormat — Select the graphic image format generated by PHPlot
$plot->SetFileFormat($format)
SetFileFormat selects a graphic image format from the
available image formats. Depending on how PHP and/or GD were built on your
system, available formats include JPEG, PNG, GIF, and WBMP.
The default file format is 'png'.
Depending on how GD was built, not all of these formats will be available.
You can use phpinfo() to see what formats are supported
by your PHP/GD installation.
JPEG is generally a bad choice for this type of image, as the lossy compression reduces the quality of lines and text.