Output Format
Image Chart API can generate multiple output image format.
Also Some software like Flowdock, Slack or Facebook messenger needs an URL that ends with a valid image extension file to display it as an image. Always explicitly set the chof parameter at the end of the URL to maximize your support of this kind of clients.
Syntax
chof=.png
Related
Output image format depends on the kind of chart generated.
Other charts output format
Other chart types like bar, line, pie charts do not depends on chof value and will output chart image in png format.
Animated charts with chan
If the chan animated chart parameter is used, no matter the value of chof the output format will always be a gif. Some email and chat clients might still need the &chof=.gif at the end of the URL to interpret it as an image.
The output format (png or gif) will not depends on the chof query parameter but instead on whether or not the chart is animated or not, see the animation section.
.../chart?chs=700x190&chd=t:60,40&cht=p3&chof=.png