[apps/graph] Put the function variable name "(x)" in function

Change-Id: I0bd287363e4fc957a72dacef1a8f61f1c276da7e

apps/graph/function.h

@@ -8,6 +8,7 @@ namespace Graph {
 
 class Function {
 public:
+  static constexpr const char * Parameter = "(x)";
   Function(const char * text = nullptr, KDColor color = KDColorBlack);
   ~Function(); // Delete expression and layout, if needed
   const char * text();