Change the canvas size, which also changes the coordinate system:
canvas.size([300, 150])
canvas.node(1).add({ pos: ['-cx+35', 'cy-35'], fixed: true })
canvas.node(2).add({ pos: ['cx-35', 'cy-35'], fixed: true })
canvas.node(3).add({ pos: ['-cx+35', '-cy+35'], fixed: true })
canvas.node(4).add({ pos: ['cx-35', '-cy+35'], fixed: true })
canvas.edges([[1, 2], [2, 4], [4, 3], [3, 1]]).add()
canvas.node('c').add({
shape: 'rect',
size: [50, 12],
pos: [0, 0],
labels: { 0: { text: 'Center (0, 0)' } }
})
Note that, in this case, the actual size of the canvas is predetermined.