By default, for security reasons, Piwik prevents standard pages and reports to be included in an iframe.
If you wish to include widgets in your website or app, we recommend you use the standard “Widgets” mechanism.
If you wish to include the whole Piwik in an iframe, edit your config/config.ini.php under the category [General] add the following line:

[General]
enable_framed_pages=1

If you also wish to enable the “Settings” pages (Manage websites, Users, etc.) to load in an iframe you can also add the setting:

enable_framed_settings=1

See also: How do I enable users to login into Piwik inside an iframe?