# Quiver Inconsistency

**URL:** <https://fenicsproject.discourse.group/t/quiver-inconsistency/2389>\
**Category:** mesh\
**Created:** [February 7, 2020, 4:20pm UTC](https://fenicsproject.discourse.group/t/quiver-inconsistency/2389 "2020-02-07T16:20:14Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![gideonsimpson](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/gideonsimpson/32/561_2.png) [@gideonsimpson](https://fenicsproject.discourse.group/u/gideonsimpson)\
**Post date:** [February 7, 2020, 4:20pm UTC](https://fenicsproject.discourse.group/t/quiver-inconsistency/2389/1 "2020-02-07T16:20:14Z")

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I’m not sure if this is merely an artifact of the plotting routine in fenics, but there’s an inconsistency I’d like to resolve. Suppose I have a donut shaped domain and I compute the \hat{\mathbf{r}} unit vector. Using `DG0` elements, I would expect these to be located at the cell centers. When I plot the function with `plot`, I see vectors at vertices, and it’s not entirely clear which ones it is choosing to plot. In contrast, if I plot, manually, using `matplotlib`'s `quiver`, I get what I expect (see figure). In the figure, I am comparing the color gradient arrows with the black and red arrows. Note that the red arrows are offset to make them more visible; they line up exactly with the black arrows. Essentially, my question is, why do the three sets not line up?

![donut](https://global.discourse-cdn.com/free1/uploads/fenicsproject1/original/1X/633916300bc248c8d8efb9411b6e92656b408944.png)

This was generated using the following code:

```auto
import numpy as np
from fenics import *
from mshr import *

from matplotlib import pyplot as plt
import sys

r0 = 0.7
r1 = 1.0
Nr = 5

mesh=generate_mesh(Circle(Point(0,0),r1)-Circle(Point(0,0),r0), Nr)
V = FunctionSpace(mesh, "DG",0)
Vvec = VectorFunctionSpace(mesh, "DG",0)
xmesh = V.tabulate_dof_coordinates()[:,0]
ymesh = V.tabulate_dof_coordinates()[:,1]

rvec = Expression(("x[0]/sqrt(x[0]*x[0]+x[1]*x[1])",
    "x[1]/sqrt(x[0]*x[0]+x[1]*x[1])"),element = Vvec.ufl_element())
r_project = np.reshape( project(rvec, Vvec).vector()[:],(47,2))
r_direct = np.asarray([[x/np.sqrt(x **2+y** 2),y/np.sqrt(x **2+y** 2)] for x,y in zip(xmesh,ymesh)])
plot(mesh)
plt.scatter(xmesh,ymesh)
plt.quiver(xmesh, ymesh, r_project[:,0], r_project[:,1],label="Projection")
plt.quiver(xmesh, ymesh, r_direct[:,0], r_direct[:,1],label="Manual",color='r',pivot='tip')
plot(project(rvec, Vvec))
plt.legend()
plt.savefig("donut.png")
plt.show()

```

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<div class="post-metadata">

**Author:** ![nate](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/nate/32/17_2.png) [@nate](https://fenicsproject.discourse.group/u/nate)\
**Post date:** [February 7, 2020, 7:23pm UTC](https://fenicsproject.discourse.group/t/quiver-inconsistency/2389/2 "2020-02-07T19:23:36Z")

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`dolfin`'s built in plotting visualises the function at the mesh vertices.

You should use your own plotting algorithm, or output your function using `XDMF.write_checkpoint` which outputs the function and its function space properties for visualisation with paraview, which handles DG{0,1,2} and CG{1,2} function spaces.

Note that `XDMF.write` will only output the evaluation of the FE function at the mesh vertices.