[apps/graph] Avoid storing weird x values in the local context

Change-Id: Iee751da03fb9c6cda2e77d2570e8af2aef5ed5da

apps/graph/app.h

@@ -15,6 +15,10 @@ class App : public Shared::TextFieldDelegateApp {
 public:
   App(Container * container, Poincare::Context * context);
   InputViewController * inputViewController();
+  /* This local context can parse x. However, it always stores NAN
+   * as x value. When we need to evaluate expression with a specific x value, we
+   * use a temporary local context (on the stack). That way, we avoid keeping
+   * weird x values after drawing curves or displaying the value table. */
   Poincare::Context * localContext() override;
 private:
   CartesianFunctionStore m_functionStore;

apps/shared/function.cpp

@@ -75,10 +75,11 @@ void Function::setActive(bool active) {
 }
 
 float Function::evaluateAtAbscissa(float x, Poincare::Context * context) const {
+  Poincare::VariableContext variableContext = Poincare::VariableContext(symbol(), context);
   Poincare::Symbol xSymbol = Poincare::Symbol(symbol());
   Poincare::Complex e = Poincare::Complex::Float(x);
-  context->setExpressionForSymbolName(&e, &xSymbol);
-  return m_expression->approximate(*context);
+  variableContext.setExpressionForSymbolName(&e, &xSymbol);
+  return m_expression->approximate(variableContext);
 }
 
 }

poincare/src/variable_context.cpp

@@ -6,7 +6,7 @@ namespace Poincare {
 
 VariableContext::VariableContext(char name, Context * parentContext) :
   m_name(name),
-  m_value(Complex::Float(0.0f)),
+  m_value(Complex::Float(NAN)),
   m_parentContext(parentContext)
 {
 }