Graphs aren’t displayed if you use Matomo Mobile 1.5 or earlier. Please update Matomo Mobile and Matomo (Piwik) server to the latest version.
If you still don’t see any graphs in Matomo Mobile ask your Administrator to open the Matomo “Plugins” page which can be found under Administration top menu link. Make sure the
ImageGraph plugin is activated. Within the description of the plugin there is also a link to verify graphs are working. Just click the “Debug: All images” link. If graphs are not displayed there, the error is caused by your Matomo installation and not by Matomo Mobile. Make sure all required extensions are installed for example the GD extension.
There may be also an issue under Android when the webserver uses gzip to deliver the images.