The plot-function you refer to was deprecated, as for most 3D visualization, Paraview is superior.
Paraview takes in either pvd or xdmf files as input.
For the problem above, saving markers as pvd function is sufficient for visualization.
Edit: You can also use VTKPlotter, see fr instance: New vtkplotter module for visualization.
Note that there have been many interface changes in the past releases, which are summarized in the Changelog.
Note that the demos have been updated reflecting these changes.
For your problem, consider the following:
from dolfin import *
import numpy as np
mesh = UnitSquareMesh(10,10)
class left_side(SubDomain):
def inside(self, x, on_boundary):
return np.isclose(x[0], 0) and on_boundary
markers = MeshFunction("size_t", mesh, mesh.topology().dim()-1, 0)
left_side().mark(markers,1)
ds = ds(domain=mesh, subdomain_data=markers)
print(assemble(1*ds), assemble(1*ds(1)))
File("markers.pvd") << markers