[exercise fluidsystem][2pproblem] add logarithmic spacing and get density from fluidsystem directly

exercises/exercise-fluidsystem/2pproblem.hh

@@ -132,6 +132,7 @@ class ExerciseFluidsystemProblemTwoP : public PorousMediumFlowProblem<TypeTag>
     using FVGridGeometry = typename GET_PROP_TYPE(TypeTag, FVGridGeometry);
     using FVElementGeometry = typename GET_PROP_TYPE(TypeTag, FVGridGeometry)::LocalView;
     using FluidSystem = typename GET_PROP_TYPE(TypeTag, FluidSystem);
+    using FluidState = typename GET_PROP_TYPE(TypeTag, FluidState);
 
     enum {
         waterPressureIdx = Indices::pressureIdx,
@@ -289,20 +290,30 @@ public:
 private:
     void plotDensity_()
     {
-        std::vector<double> x(100);
-        std::vector<double> y(100);
-        for (int i=0; i<100; ++i)
-            x[i] = 1e4 + i*100910;
-        for (int i=0; i<100; ++i)
-            y[i] = MyCompressibleComponent<Scalar>::liquidDensity(300, x[i]);
+        FluidState fluidState;
+        fluidState.setTemperature(temperature());
+        int numberOfPoints = 100;
+        Scalar xMin = 1e4;
+        Scalar xMax = 1e7;
+        Scalar spacing = std::pow((xMax/xMin), 1.0/(numberOfPoints-1));
+        std::vector<double> x(numberOfPoints);
+        std::vector<double> y(numberOfPoints);
+        for (int i=0; i<numberOfPoints; ++i)
+            x[i] = xMin*std::pow(spacing,i);
+        for (int i=0; i<numberOfPoints; ++i)
+        {
+            fluidState.setPressure(FluidSystem::phase1Idx, x[i]);
+            y[i] = FluidSystem::density(fluidState, FluidSystem::phase1Idx);
+        }
         gnuplot_.resetPlot();
-        gnuplot_.setXRange(1e4, 1e7);
+        gnuplot_.setXRange(xMin, xMax);
         gnuplot_.setOption("set logscale x 10");
+        gnuplot_.setOption("set key left top");
         gnuplot_.setYRange(1440, 1480);
         gnuplot_.setXlabel("pressure [Pa]");
         gnuplot_.setYlabel("density [kg/m^3]");
-        gnuplot_.addDataSetToPlot(x, y, "YourComponent_density.dat", "w l t 'Liquid density of your component'");
-        gnuplot_.plot("YourComponent_density");
+        gnuplot_.addDataSetToPlot(x, y, "YourComponentPhase_density.dat", "w l t 'Phase consisting of your component'");
+        gnuplot_.plot("YourComponentPhase_density");
     }
 
     Scalar eps_; //! small epsilon value