CorePlot - Set y-Axis range to include all data points

I have implemented a CorePlot graph in my iOS application, however I am unable to dynamically size the y-Axis based on the data points in the set. Some datasets range from 10-50, while others would range from 400-500. In both cases, I would like to have y-origin (0) visible.

I have tried using the scaletofitplots method:

[graph.defaultPlotSpace scaleToFitPlots:[graph allPlots]];

but this has no effect on the graphs, they still show the default Y-Axis range of 0 to 1.

The only way that I can change the y-Axis is to do it manually via this:

graph.defaultPlotSpace.yRange = [CPTPlotRange plotRangeWithLocation:CPTDecimalFromFloat(0.0f)) length:CPTDecimalFromFloat(500.0f)];

however this doesn't work well for graphs with smaller ranges (10-50), as you can imagine.

Is there any method to retrieve the highest y-value from my dataset so that I can manually set the maximum y-Axis to that value? Or is there a better alternative?

EDIT : Here's the file I am using:

#import "TUTSimpleScatterPlot.h"

@implementation TUTSimpleScatterPlot

@synthesize hostingView = _hostingView;
@synthesize graph = _graph;
@synthesize graphData = _graphData;

-(id)initWithHostingView:(CPTGraphHostingView *)hostingView andData:(NSMutableArray *)data {
    self = [super init];

    if (self != nil) {
        self.hostingView = hostingView;
        self.graphData = data;
        self.graph = nil;
    }
    return self;
}

-(void)initialisePlot {

    if ((self.hostingView == nil) || (self.graphData == nil)) {
        NSLog(@"TUTSimpleScatterPlot: Cannot Initialise plot with hosting view and data");
        return;
    }

    if (self.graph != nil) {
        NSLog(@"TUTSimpleScatterPlot: Graph Object already exists.");
    }
    NSDate *refDate = [NSDate date];
    NSTimeInterval oneDay = 24 * 60 * 60;

    CGRect frame = [self.hostingView bounds];
    self.graph = [[CPTXYGraph alloc] initWithFrame:frame];

    CPTXYPlotSpace *plotSpace = (CPTXYPlotSpace *)self.graph.defaultPlotSpace;
    plotSpace.allowsUserInteraction = YES;
    plotSpace.delegate = self;
    self.graph.plotAreaFrame.paddingBottom = 20.0f;
    self.graph.plotAreaFrame.paddingLeft = 35.0f;


    self.hostingView.hostedGraph = self.graph;

    CPTMutableLineStyle *lineStyle = [CPTMutableLineStyle lineStyle];
    lineStyle.lineColor = [CPTColor whiteColor];
    lineStyle.lineWidth = 2.0f;

    CPTMutableLineStyle *lineStyleThin = [CPTMutableLineStyle lineStyle];
    lineStyleThin.lineColor = [CPTColor whiteColor];
    lineStyleThin.lineWidth = 1.0f;

    CPTMutableTextStyle *textStyle = [CPTMutableTextStyle textStyle];
    textStyle.fontName = @"HelveticaNeue-Medium";
    textStyle.fontSize = 10;
    textStyle.color = [CPTColor whiteColor];

    CPTMutableLineStyle *plotSymbolLineStyle = [CPTMutableLineStyle lineStyle];
    plotSymbolLineStyle.lineColor = [CPTColor whiteColor];
    plotSymbolLineStyle.lineWidth = 1.0f;

    CPTPlotSymbol *plotSymbol = [CPTPlotSymbol ellipsePlotSymbol];
    plotSymbol.lineStyle = plotSymbolLineStyle;
    plotSymbol.fill = [CPTFill fillWithColor:[CPTColor colorWithComponentRed:0.4 green:0.6 blue:0.8 alpha:1.0]];
    plotSymbol.size = CGSizeMake(8.0, 8.0);

    CPTScatterPlot *plot = [[CPTScatterPlot alloc] init];
    plot.dataSource = self;
    plot.identifier = @"mainPlot";

    [plotSpace scaleToFitPlots:[graph allPlots]];

    CPTMutablePlotRange *yRange = [plotSpace.yRange mutableCopy];
    [yRange expandRangeByFactor:CPTDecimalFromDouble(615)];
    plotSpace.yRange = yRange;

    plotSpace.xRange = [CPTPlotRange plotRangeWithLocation:CPTDecimalFromFloat(oneDay - (oneDay * 6.3f)) length:CPTDecimalFromFloat(oneDay * 6.6f)];
    plotSpace.allowsUserInteraction = YES;

    CPTPlotRange *globalYRange = [CPTPlotRange plotRangeWithLocation:CPTDecimalFromDouble(0.0f) length:CPTDecimalFromDouble(615.0f)];
    plotSpace.globalYRange = globalYRange;

    // Modify the graph's axis with a label, line style, etc
    CPTXYAxisSet *axisSet = (CPTXYAxisSet *)self.graph.axisSet;

    axisSet.xAxis.axisConstraints = [CPTConstraints constraintWithLowerOffset:0.0]; 
    axisSet.xAxis.axisLineStyle = lineStyle;
    axisSet.xAxis.majorTickLineStyle = lineStyle;
    axisSet.xAxis.labelTextStyle = textStyle;
    axisSet.xAxis.labelOffset = 1.0f;
    axisSet.xAxis.majorIntervalLength = CPTDecimalFromFloat(oneDay*1);
    axisSet.xAxis.orthogonalCoordinateDecimal = CPTDecimalFromString(@"0.0");
    axisSet.xAxis.minorTicksPerInterval = 0;
    axisSet.xAxis.majorTickLength = 0.0f;
    axisSet.xAxis.majorGridLineStyle = majorGridLineStyle;

    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:@"dd MMM"];
    CPTTimeFormatter *timeFormatter = [[CPTTimeFormatter alloc] initWithDateFormatter:dateFormatter];
    timeFormatter.referenceDate = refDate;
    axisSet.xAxis.labelFormatter = timeFormatter;

    axisSet.yAxis.axisLineStyle = lineStyle;
    axisSet.yAxis.majorTickLineStyle = lineStyle;
    axisSet.yAxis.minorTickLineStyle = lineStyleThin;
    axisSet.yAxis.labelTextStyle = textStyle;
    axisSet.yAxis.labelOffset = 3.0f;
    axisSet.yAxis.majorIntervalLength = CPTDecimalFromString(@"50.0");
    axisSet.yAxis.minorTicksPerInterval = 1;
    axisSet.yAxis.minorTickLength = 3.0;
    axisSet.yAxis.majorTickLength = 5.0;
    axisSet.yAxis.orthogonalCoordinateDecimal = CPTDecimalFromFloat(0);
    axisSet.yAxis.majorGridLineStyle = majorGridLineStyle;

    axisSet.yAxis.axisConstraints = [CPTConstraints constraintWithLowerOffset:0.0];

    CPTMutableLineStyle *theLineStyle = [CPTMutableLineStyle lineStyle];
    theLineStyle.lineColor = [CPTColor  colorWithComponentRed:0.3 green:0.6 blue:0.9 alpha:1.0];
    theLineStyle.lineWidth = 2.0f;

    plot.dataLineStyle = theLineStyle;
    plot.plotSymbol = plotSymbol;

    [self.graph addPlot:plot];

}

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