Target/limiter plates are plotted half a grid point off from where they should be

[johnomotani]

At the moment when the targets are plotted, they are drawn along the line of the first/last grid points in the y-direction.

xBOUT/xbout/plotting/utils.py

Lines 124 to 137 in 492d0a3

	if list(da_region.regions.values())[0].connection_lower_y is None:
	# lower target exists
	x_target = da_region.coords[x].isel(**{ycoord: y_boundary_guards})
	y_target = da_region.coords[y].isel(**{ycoord: y_boundary_guards})
	[line] = ax.plot(x_target, y_target, "k-", linewidth=2)
	if hatching:
	_add_hatching(line, ax)
	if list(da_region.regions.values())[0].connection_upper_y is None:
	# upper target exists
	x_target = da_region.coords[x].isel(**{ycoord: -y_boundary_guards - 1})
	y_target = da_region.coords[y].isel(**{ycoord: -y_boundary_guards - 1})
	[line] = ax.plot(x_target, y_target, "k-", linewidth=2)
	if hatching:
	_add_hatching(line, ax, reversed=True)

They should really be half way between the end-most grid point and the first boundary point (to be consistent with BOUT++ boundary conditions definitions). This should be straightforward if we have the y boundary cells, but requires an extrapolation if they are not present.