Return 1 if hold is on.
ISHOLD returns 1 if hold is on, and 0 if it is off.
When HOLD is ON, the current plot and all axis properties
are held so that subsequent graphing commands add to the
Hold on means the NextPlot property of both figure
and axes is set to "add".
See also HOLD10XXCU2, NEWPLOT , FIGURE , AXES .