minor code cleaning and examples update

examples/exPlot.cpp

@@ -0,0 +1,16 @@
+#include <iostream>
+#include <latan/Plot.hpp>
+
+using namespace std;
+using namespace Latan;
+
+int main(void)
+{
+    Plot p;
+    
+    p << PlotCommand("x**2") << PlotCommand("x**3") << PlotCommand("x**4");
+    cout << p << endl;
+    p.display();
+    
+    return EXIT_SUCCESS;
+}