def y_range
max_value = @data.collect{|x| x[:data][Y].max }.max
min_value = @data.collect{|x| x[:data][Y].min }.min
min_value = min_value<min_y_value ? min_value : min_y_value if min_y_value
range = max_value - min_value
top_pad = range == 0 ? 10 : range / 20.0
scale_range = (max_value + top_pad) - min_value
scale_division = scale_y_divisions || (scale_range / 10.0)
if scale_y_integers
scale_division = scale_division < 1 ? 1 : scale_division.round
end
return [min_value, max_value, scale_division]
end