Creating a graph in Objective-C on the iPhone

// Render a graph of the given width and height and return it as an image. Main thread only.
- (UIImage*)renderGraph:(int)w h:(int)h {
    // Figure out the font and it's metrics
    UIFont* font = [UIFont fontWithName:@"AUdimat" size:10];
    int fontHeight = [@"ABC123" sizeWithFont:font].height;
    
    // Constants
    UIColor *lengthCol = [UIColor redColor];
    UIColor *gapCol = [UIColor greenColor];
    UIColor *rangeCol = [UIColor whiteColor];
    int ranges = 9; // Number of points on the Y axis scale
    int pad = 7; // How far to pad in the edges
    int topCount = 15; // How many data points to show
    int rangeWidth = 45; // How wide is the y-axis range labels
    int labelWidth=rangeWidth-10, dashWidth=4, dashInset=rangeWidth-3; // Widths for the y-axis range parts
    
    // Begin the graphics context
    UIGraphicsBeginImageContextWithOptions(CGSizeMake(w, h), NO, [[UIScreen mainScreen] scale]);
    CGContextRef context = UIGraphicsGetCurrentContext();
    
    // Look after padding
    CGContextTranslateCTM(context, pad, pad); // Pad everything by a bit
    w-=pad*2; h-=pad*2; // Shrink the size so it thinks it's 10px smaller due to padding
    
    // Do we have enough data to plot?
    if (self.contractions.count<2) {
        [[UIColor grayColor] setFill];
        [@"Waiting for data" drawInRect:CGRectMake(0, h/2-fontHeight/2, w, fontHeight)
                               withFont:font lineBreakMode:(UILineBreakModeClip) alignment:(UITextAlignmentCenter)];
    } else { // We *do* have enough data
        // Get the top 15 data points (or all points if <= 15)
        NSArray* topOnes = self.contractions.count > topCount ?
            [self.contractions subarrayWithRange:NSMakeRange(self.contractions.count-topCount, topCount)] :
            self.contractions;
        
        // Find the maximum scale for the y-axis
        int maxScale=0;
        for (Contraction* c in topOnes) { // Find the max lengths of the contractions
            maxScale = MAX(maxScale, (int)round(c.length));
        }
        for (int i=0; i<topOnes.count-1; i++) { // Find the max gap between contractions
            Contraction* c1 = [topOnes objectAtIndex:i];
            Contraction* c2 = [topOnes objectAtIndex:i+1];
            int gapSeconds = round([c2.start timeIntervalSinceDate:c1.start]);
            maxScale = MAX(maxScale, gapSeconds);
        }
        
        // Tweak the scale so that the gaps between the y-axis range lines are sensible
        int rangeGap = maxScale/(ranges-1); // Gap between range lines
        if (rangeGap<15) rangeGap=15;
        else if (rangeGap<30) rangeGap=30;
        else if (rangeGap<5*60) rangeGap=ceil(rangeGap/60.0)*60.0; // Every minute up to 5 mins
        else if (rangeGap<10*60) rangeGap=10*60; // 10 min gap
        else if (rangeGap<15*60) rangeGap=15*60; // 15 min gap
        else if (rangeGap<30*60) rangeGap=30*60; // 30 min gap
        else rangeGap=ceil(rangeGap/3600.0)*3600.0; // Hourly gaps
        maxScale = rangeGap*(ranges-1); // Convert from gap back to max scale again
        
        // Draw the y-axis range lines and labels
        [rangeCol setFill];
        [rangeCol setStroke];
        for (int range=0; range<ranges; range++) {
            int thisRangeY = h-h*range/(ranges-1);
            [self line:w-dashInset y:thisRangeY x2:w-dashInset+dashWidth y2:thisRangeY];
            [[self hms:maxScale*range/(ranges-1)] drawInRect:CGRectMake(w-labelWidth, thisRangeY-fontHeight/2, labelWidth, fontHeight)
                                                    withFont:font lineBreakMode:UILineBreakModeTailTruncation alignment:UITextAlignmentLeft];
        }
        
        // Draw the labels for the series
        [lengthCol setFill];
        [@"Duration" drawInRect:CGRectMake(w/2-105, 0, 100, fontHeight) withFont:font lineBreakMode:UILineBreakModeClip alignment:UITextAlignmentRight];
        [gapCol setFill];
        [@"Frequency" drawInRect:CGRectMake(w/2+5, 0, 100, fontHeight) withFont:font lineBreakMode:UILineBreakModeClip alignment:UITextAlignmentLeft];
        
        // Draw the graph for contraction lengths
        int cWid = (w-rangeWidth)/(topOnes.count-1);
        {
            [lengthCol setStroke];
            int i=0;
            int lastX=-1, lastY=-1;
            for (Contraction* c in topOnes) {
                int thisX = i*cWid;
                int thisY = h-h*round(c.length)/maxScale;
                [self dot:thisX y:thisY];
                if (lastX>=0) {
                    [self line:lastX y:lastY x2:thisX y2:thisY];
                }
                i++;
                lastX = thisX;
                lastY = thisY;
            }
        }
        
        // Draw the graph for contraction ggaps
        {
            [gapCol setStroke];
            int lastX=-1, lastY=-1;
            for (int i=0; i<topOnes.count-1; i++) {
                Contraction* c1 = [topOnes objectAtIndex:i];
                Contraction* c2 = [topOnes objectAtIndex:i+1];
                int gapSeconds = round([c2.start timeIntervalSinceDate:c1.start]);

                int thisX = i*cWid + cWid/2;
                int thisY = h-h*gapSeconds/maxScale;
                [self dot:thisX y:thisY];
                if (lastX>=0) {
                    [self line:lastX y:lastY x2:thisX y2:thisY];
                }
                lastX = thisX;
                lastY = thisY;
            }
        }
    }
        
    UIImage *img = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return img;
}

// Draw a line from point to point
- (void)line:(int)x y:(int)y x2:(int)x2 y2:(int)y2 {
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextBeginPath(context);
    CGContextMoveToPoint(context, x, y);
    CGContextAddLineToPoint(context, x2, y2);
    CGContextStrokePath(context);
}

// Draw a dot at a certain point by doing a line with round caps
- (void)dot:(int)x y:(int)y {
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSaveGState(context); // So the setlinecap+setlinewidth don't leak out beyond this function
    CGContextSetLineCap(context, kCGLineCapRound);
    CGContextSetLineWidth(context, 5);
    CGContextBeginPath(context);
    CGContextMoveToPoint(context, x, y);
    CGContextAddLineToPoint(context, x, y);
    CGContextStrokePath(context);
    CGContextRestoreGState(context);
}