System Load Indicator only shows as a line unless hovered

Although System Load Indicator (indicator-multiload) works well with desktop environments like Unity, MATE etc., it's not very compatible with GNOME 3 (default in Ubuntu 17.10 and later).

If you want to try an alternative solution which is compatible with GNOME, you may use a GNOME shell extension called "system-monitor". It does

Display system informations in gnome shell status bar, such as memory usage, cpu usage, network rates…

You can also install it from the 'Software' application or by running

sudo apt install gnome-shell-extension-system-monitor