JConsole accepts negative numbers and zero:

% jconsole -interval=-123

It would be better to check if interval > 0.
When started with interval <=0, JConsole doesn't display graps at all and prints an exception:

Exception in thread "AWT-EventQueue-0" java.lang.IllegalArgumentException: Non-positive period.
        at java.util.Timer.schedule(Timer.java:221)
        at sun.tools.jconsole.VMPanel.startUpdateTimer(VMPanel.java:442)
        at sun.tools.jconsole.VMPanel.propertyChange(VMPanel.java:321)
        at java.beans.PropertyChangeSupport.firePropertyChange(PropertyChangeSupport.java:339)
        at javax.swing.event.SwingPropertyChangeSupport.firePropertyChange(SwingPropertyChangeSupport.java:75)
        at javax.swing.event.SwingPropertyChangeSupport$1.run(SwingPropertyChangeSupport.java:80)
        at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:597)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)
Sorry, this is a duplicate. Closing.