[vis] Fix stream plots, only use for non-zero fields

phi/vis/_matplotlib/_matplotlib_plots.py

@@ -431,7 +431,7 @@ def plot(self, data: Field, figure, subplot, space: Box, min_val: float, max_val
 class StreamPlot2D(Recipe):
 
     def can_plot(self, data: Field, space: Box) -> bool:
-        return data.spatial_rank == 2 and 'vector' in channel(data) and data.is_grid
+        return data.spatial_rank == 2 and 'vector' in channel(data) and data.is_grid and (data.values != 0).any
 
     def plot(self, data: Field, figure, subplot, space: Box, min_val: float, max_val: float, show_color_bar: bool, color: Tensor, alpha: Tensor, err: Tensor):
         vector = data.geometry.shape['vector']
@@ -441,14 +441,14 @@ def plot(self, data: Field, figure, subplot, space: Box, min_val: float, max_val
         y = y[0, :]
         u, v = reshaped_numpy(data.values, [vector, *data.shape.without('vector')])
         if (color == None).all:
-            col = reshaped_numpy(math.vec_length(data.values), [*data.shape.without('vector')])
+            col = reshaped_numpy(math.vec_length(data.values), [*data.shape.without('vector')]).T
         else:
             if color.shape:
                 col = [_plt_col(c) for c in color.numpy(data.shape.non_channel).reshape(-1)]
             else:
                 col = _plt_col(color)
         # alphas = reshaped_numpy(alpha, [data.shape.without('vector')])  alpha not supported yet
-        subplot.streamplot(x, y, u, v, color=col, cmap=plt.cm.get_cmap('viridis'))
+        subplot.streamplot(x, y, u.T, v.T, color=col, cmap=plt.cm.get_cmap('viridis'))
 
 
 class EmbeddedPoint2D(Recipe):