[apps/graph] Change name: function->cartesian_function

Change-Id: I138bd13fec8514144cfde7164ec9aff081454609

apps/graph/cartesian_function.cpp

@@ -0,0 +1,31 @@
+#include "cartesian_function.h"
+
+namespace Graph {
+
+CartesianFunction::CartesianFunction(const char * text, KDColor color) :
+  Shared::Function(text, color),
+  m_displayDerivative(false)
+{
+}
+
+bool CartesianFunction::displayDerivative() {
+  return m_displayDerivative;
+}
+
+void CartesianFunction::setDisplayDerivative(bool display) {
+  m_displayDerivative = display;
+}
+
+float CartesianFunction::approximateDerivative(float x, Poincare::Context * context) const {
+  Poincare::Complex abscissa = Poincare::Complex::Float(x);
+  Poincare::Expression * args[2] = {m_expression, &abscissa};
+  Poincare::Derivative derivative = Poincare::Derivative();
+  derivative.setArgument(args, 2, true);
+  return derivative.approximate(*context);
+}
+
+char CartesianFunction::symbol() const {
+  return 'x';
+}
+
+}