Add tracers example in cookbook

Author: amorison

docs/index.rst

@@ -25,6 +25,7 @@ Welcome to StagPy's documentation!
    sources/cookbook/field
    sources/cookbook/nura
    sources/cookbook/vrmsra
+   sources/cookbook/tracers
 
 .. toctree::
    :maxdepth: 2

docs/sources/cookbook/tracers.rst

@@ -0,0 +1,32 @@
+Plotting tracers information
+============================
+
+StagPy gives you access to tracers information through StagyyData. Mind that
+tracers information is organized by blocks even if your run only has one
+block. The following script offers an example::
+
+    from stagpy.stagyydata import StagyyData
+    import matplotlib.pyplot as plt
+    import numpy as np
+
+    sdat = StagyyData('.')
+
+    time = []
+    energy = []
+    for snap in sdat.snaps:
+        time.append(snap.time)
+        energy.append(np.sum(snap.tracers['Mass'][0] *
+                             snap.tracers['Temperature'][0]))
+
+    fig, axes = plt.subplots(nrows=2)
+
+    axes[0].plot(time, energy)
+    axes[0].set_xlabel("Time")
+    axes[0].set_ylabel("Internal energy")
+
+    axes[1].hist(sdat.snaps[-1].tracers['TimeMelted'][0])
+    axes[1].set_xlabel("Time melted")
+    axes[1].set_ylabel("Number of tracers")
+
+    fig.tight_layout()
+    plt.savefig('tracers.pdf')