Trac #10980: Make sure symbolic gridline values are okay

From [http://ask.sagemath.org/question/454/can-gridlines-be-painted-at-
sqrt2 this ask.sagemath.org question].
{{{
plot(x,0,2,gridlines=([sqrt(2)],[]))
}}}
blows up.

The fix is probably to make sure that symbolic input with a `n()` method
passes something right to matplotlib, which of course cannot handle
`sqrt(2)`.  Possibly beginner ticket.

Note: A beginner who is interested in working on this ticket can skip
down to comment:13.

URL: https://trac.sagemath.org/10980
Reported by: kcrisman
Ticket author(s): Atte Niemi
Reviewer(s): Dave Morris

Author: Release Manager

src/sage/plot/plot.py

@@ -518,6 +518,11 @@ def f(x): return (x-3)*(x-5)*(x-7)+40
     sage: os.system("sage -c \"if 'matplotlib' in sys.modules: sys.exit(1)\"") # long time
     0
 
+Verify that :trac:`10980` is fixed::
+
+    sage: plot(x,0,2,gridlines=([sqrt(2)],[]))
+    Graphics object consisting of 1 graphics primitive
+
 AUTHORS:
 
 - Alex Clemesha and William Stein (2006-04-10): initial version