Deboxed, axes and labels in a linear programming 3Dgraphic

edit

Unfortunately, the set_axes_labels function from sage.manifolds.utilities uses a convention different from axes, so that the x_2 label is ill-placed. However it is easy to adapt it, from the source code that you get by typing set_axes_labels??. Replace the last two lines of the code snippet of your example by

def set_axes_labels(graph, xlabel, ylabel, zlabel, **kwds):
    from sage.plot.plot3d.shapes2 import text3d
    xmin, ymin, zmin = graph.bounding_box()[0]
    xmax, ymax, zmax = graph.bounding_box()[1]
    dx = xmax - xmin
    dy = ymax - ymin
    dz = zmax - zmin
    x1 = xmin + dx / 2
    y1 = ymin + dy / 2
    z1 = zmin + dz / 2
    xmin1 = xmin - dx / 20
    xmax1 = xmax + dx / 20
    ymin1 = ymin - dy / 20
    zmin1 = zmin - dz / 20
    graph += text3d('  ' + xlabel, (x1, ymin1, zmin1), **kwds)
    graph += text3d('  ' + ylabel, (xmin1, y1, zmin1), **kwds)
    graph += text3d('  ' + zlabel, (xmin1, ymin1, z1), **kwds)
    return graph

plot = plane + pol + axes(1, color='black')
plot = set_axes_labels(plot, 'x_1','x_2','x_3')
plot

You can also replace the last line by

show(plot, viewer='threejs')

in order to use the three.js viewer (to become the default in SageMath 9.0).