The ICU pattern format groups 0 in with the positive values, there is no way (using the format option) to treat 0 differently. If you need to format it differently, you need to use the vAxis.ticks option, which takes an array of values or objects. Values specify the values to put a tick mark at. Objects have "v" and "f" properties, where the "v" property is the value to place the tick mark at and the "f" property is the string to use for the label.
vAxis: {
ticks: [{v: -3, f: '-3'}, {v: -2, f: '-2'}, {v: -1, f: '-1'}, {v: 0, f: '0'}, {v: 1, f: '+1'}, {v: 2, f: '+2'}, {v: 3, f: '+3'}]
}
Using the ticks option overrides all of the default tick mark calculations, so you must specify all of your tick marks on the option.