Hi all! I’m attempting to visualize a 3rd-order Lagrange function over the unit square in Paraview with `XDMFFile.write_checkpoint()`

. This works as expected for linear and quadratic elements, but not for 3rd, 4th, 5th, etc-order elements. This functionality is supported by Paraview (>=5.5.0) as best I can tell. See the MWE below:

```
from dolfin import *
mesh = UnitSquareMesh(1,1)
k = 3 # high-order visualization is linear with k>2
V = FunctionSpace(mesh,"Lagrange",k)
expr = Expression("x[0]*x[0]*x[1]*x[1]",degree=2*k)
u = interpolate(expr,V)
u.rename("u","u")
XDMFFile("u_linear.xdmf").write(u)
XDMFFile("u_high-order.xdmf").write_checkpoint(u,"u_high_order")
```

Here is a sample plot with `k = 2`

, which works as expected:

Here is a sample with `k = 3`

, which does not work as expected:

I am using the following:

- FEniCS 2019.0.1 installed on Docker
- Paraview 5.8.1 installed on Windows 10 from Download | ParaView

Thanks in advance for your assistance!