gnuplot if else

gnuplot 4.4 and lower:

 if (<condition>) <command-line> [; else if (<condition>) ...; else ...]

commands are just written after if condition

gnuplot 4.6

 if (<condition>) { <command>; <command>  
  <commands>  
  <commands>  
  } else {  
  <commands>   
  }  

 Commands enclosed in curly brackets.

string comparison:

test eq "test" 

And with this framework you can start building your logical setups in gnuplot, have fun.