BUG: Fix streamplot when velocity component is exactly zero.

Author: tonysyu

lib/matplotlib/streamplot.py

@@ -461,14 +461,14 @@ def _euler_step(xf_traj, yf_traj, dmap, f):
     xi = xf_traj[-1]
     yi = yf_traj[-1]
     cx, cy = f(xi, yi)
-    if cx > 0:
-        dsx = (nx - 1 - xi) / cx
-    else:
+    if cx < 0:
         dsx = xi / -cx
-    if cy > 0:
-        dsy = (ny - 1 - yi) / cy
     else:
+        dsx = (nx - 1 - xi) / cx
+    if cy < 0:
         dsy = yi / -cy
+    else:
+        dsy = (ny - 1 - yi) / cy
     ds = min(dsx, dsy)
     xf_traj.append(xi + cx*ds)
     yf_traj.append(yi + cy*ds)