.animate with a += variable value?

.animate with a += variable value?

Good day!
I'm getting my teeth into jQuery and I'm trying to add a variable Number to an objects position, at the moment I can only add a Number or use an = variable value. This is my working code:
  1. var xMove = 200;
  2. var speed = 2000;

  4.     $("#clickhere").click(function() { 
  5.         $("#animate1").animate({ left: xMove }, speed);  
  6.     }); 
  7. }); 

But I want something that works like this:

  1. var xMove = 200;
  2. var speed = 2000;

  4.     $("#clickhere").click(function() { 
  5.         $("#animate1").animate({ left: +=xMove }, speed);  
  6.     }); 
  7. }); 


Like I said using a fixed Number instead of a variable works too so it seems like it should be straighforward.
Is my syntax just wrong or is it not that simple?
Big thanks, Jack