The sparklines (small graphs) are not showing in my Matomo.

The GD library is not installed on your server. This library is required to produce the sparklines. Please ask your system administrator or your web host to install php-gd for you. For more information, check the Matomo Diagnostics Check.

On linux, you can install php-gd by executing apt-get install php-gd on debian type systems, then restart your webserver.

If you are using Apache webserver and a module called mod_security, you may need to disable this mod_security module as it may sometimes cause Sparklines graphs to not display.