Set the vAxis.maxValue and vAxis.minValue parameters. Assuming maxValue is greater than the largest value in the chart and minValue is less than the smallest value in the chart, the API will use these numbers to determine the maximum and minimum axis values. The API does not, however, use these values explicitly, it interpolates its own values (ie, max is always >= the larger of maxValue or the maximum value).
If you need specific axis values that the API does not choose, you can use the vAxis.viewWindow.max and vAxis.viewWindow.min options. These are more like zoom settings than axis settings, though, so they may not work perfectly for your needs.