Question: An empty graph for plotting

Hi!

I am trying to plot points, which are supposed to be connected by splines. Within a range of say i from 1 to 5 I would like to calculate f(i) and plot the results. It is however possible in my case that the result is discarded before, lets assume because the result is complex. Then no point is saved for that particular i and the program goes on to i+1. It is then possible to have no point and no spline defined. Therefore the function "plot" can complain if the used names for the graphs have not been defined before, because there is nothing to define. Is there a way to define an empty graph, so that I can define the graphs as the empty graph in the beginning, so that the plot command doesn't complain if no usable points are found? I would need something like this:

Graph:= <EmptyGraph>;
Spline:=<EmptyGraph>;
if numelems(<ListWithUsablePoints>) > 0  then
  <define(Graph)>;
  if numelems(<ListWithUsablePoints>) > 1 then
     <define(Spline)>
  fi;
fi;
plot(Graph, Spline,...)

If the condition under which the points are used is never met, then plot should not complain about Graph and Spline not being defined.

Thank you in advance :)