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 php5-gd for you.
On linux, you can install php-gd by executing apt-get install php5-gd on debian type systems, then restart your webserver.
If you are using Apache webserver and a module called mod_security, please disable this mod_security module as it may sometimes cause Sparklines graphs to not display.