For a pie chart you can use json.hideLabelsPercent = 5 to exclude labels for values less than 5 percent.
I’m not entirely sure what you are aiming for with bar charts, but the value axis can be controlled through json.valueAxes. For example, the following ensures the axis range is from (0, 20):
json.valueAxes = [{
minimum: 0,
maximum: 20
}];
If you would like to experiment with other settings, see:
Actually, I do have one more question. I haven’t been able to find the answer. In the legend of a chart, is there a way to ‘hide’ anything equal to zero?