iOS arrow drawing code, ported to MonoTouch / C# from https://gist.github.com/mayoff/4146780

CreateArrowPath.cs

using System;
using System.Drawing;
using MonoTouch.CoreGraphics;

/// https://gist.github.com/mayoff/4146780

static class ArrowPath
{
	public static CGPath pathWithArrowFromPoint(
		PointF startPoint,
		PointF endPoint,
		float tailWidth,
		float headWidth,
		float headLength)
	{
		var dx = endPoint.X - startPoint.X;
		var dy = endPoint.Y - startPoint.Y;
		var length = (float)Math.Sqrt(dx*dx + dy*dy);
		var points = getAxisAlignedArrowPoints(length, tailWidth, headWidth, headLength);
		var transform = transformForStartPoint(startPoint, endPoint, length);

		var path = new CGPath();
		path.AddLines(transform, points);
		path.CloseSubpath();

		return path;
	}

	static PointF[] getAxisAlignedArrowPoints(
		float length,
		float tailWidth, 
		float headWidth, 
		float headLength)
	{
		var tailLength = length - headLength;
		var points = new PointF[7];

		points[0] = new PointF(0, tailWidth / 2);
		points[1] = new PointF(tailLength, tailWidth / 2);
		points[2] = new PointF(tailLength, headWidth / 2);
		points[3] = new PointF(length, 0);
		points[4] = new PointF(tailLength, -headWidth / 2);
		points[5] = new PointF(tailLength, -tailWidth / 2);
		points[6] = new PointF(0, -tailWidth / 2);

		return points;
	}

	static CGAffineTransform transformForStartPoint(PointF startPoint, PointF endPoint, float length)
	{
		var cosine = (endPoint.X - startPoint.X) / length;
		var sine = (endPoint.Y - startPoint.Y) / length;
		return new CGAffineTransform(cosine, sine, -sine, cosine, startPoint.X, startPoint.Y);
	}
}