You can enable zooming on an axis like so.
_chart.xAxis.enableGestureZooming = YES;
I'm not sure I'm following what you mean when you say the bar is too high. It seems to me as though you might be saying that you Y-axis data range is too small? If so you could add some padding to it.
_chart.yAxis.rangePaddingHigh = @200;
Or you could set your Y-axis range manually when you create it.
SChartNumberRange *range = [[SChartNumberRange alloc] initWithMinimum:@0 andMaximum:@500];
_chart.yAxis = [[SChartNumberAxis alloc] initWithRange:range];