Merge branch 'dailywork-ftschindler' into 'master'

[P|grid] allow to visualize 1d grid

See merge request !71

python/dune/xt/grid/__init__.py

@@ -11,6 +11,7 @@
 #   Tobias Leibner  (2019 - 2020)
 # ~~~
 
+import numpy as np
 from numbers import Number
 from tempfile import NamedTemporaryFile
 
@@ -48,7 +49,27 @@ def Dim(d):
 
 
 def visualize_grid(grid):
-    tmpfile = NamedTemporaryFile(mode='wb', delete=False, suffix='.vtu').name
-    grid.visualize(tmpfile[:-4])
-    return plot(
-        tmpfile, color_attribute_name='Element index')     # see visualize in python/dune/xt/grid/gridprovider.hh
+    if grid.dimension == 1:
+        from matplotlib import pyplot as plt
+
+        centers = np.array(grid.centers(1), copy=False)[:, 0]
+
+        points = np.zeros(2*grid.size(0))
+        points[1::2] = centers[1:]
+        points[2::2] = centers[1:(len(centers) - 1)]
+
+        indices = np.zeros(2*grid.size(0))
+        indices[::2] = np.arange(grid.size(0))
+        indices[1::2] = np.arange(grid.size(0))
+
+        plt.figure()
+        plt.title(f'grid with {grid.size(0)} elements')
+        plt.plot(points, indices)
+
+        return plt.gca()
+
+    else:
+        tmpfile = NamedTemporaryFile(mode='wb', delete=False, suffix='.vtu').name
+        grid.visualize(tmpfile[:-4])
+        return plot(
+            tmpfile, color_attribute_name='Element index')     # see visualize in python/dune/xt/grid/gridprovider.hh