Fixed bug with "Change Plot Limits"

When a limit was set to 0, it wasn’t changed.
Fixed setting of 'axes' in SignalDialog

The current procedure is to set ‘signal’ and ‘axes’ as both  group and
field attributes, but SignalDialog only set the group’s ‘signal’
attribute.
Fixed PlotDialog bug

After refactoring, the node.plot format argument has to be set as a
keyword, ‘fmt’.