${!} an alternate syntax for $! which is the PID of the last background command (presumably here, your graph application). The script simply waits for the application to finish.

Generally, ${anything} is just another way to write $anything -- in some situations, the braces are required for disambiguation (${anything}too means something else than $anythingtoo which means ${anythingtoo}) but some people like to put them everywhere just out of habit, or for clarity.