Histogram image in C#

gistfile1.cs

public static class Histogram
{
    public static SortedDictionary<int, int> ToHistogram(this IEnumerable<int> nums)
    {
        var dict = new SortedDictionary<int, int>();
        foreach (var n in nums)
        {
            if (!dict.ContainsKey(n))
                dict[n] = 1;
            else
                dict[n] += 1;
        }
        return dict;
    }
    
    public static Bitmap ToBitmap(this IEnumerable<KeyValuePair<int, int>> histogram, float heightScale = 3.0f, int xLabelOffset = 12, int yLabelOffset = 4)
    {
        var marginTop = 5;
        var marginBottom = 50;
        var marginLeft = 50;
    
        var max = histogram.Max(kvp => kvp.Key);
        var min = histogram.Min(kvp => kvp.Key);
        var width = max - min;
        var maxValue =  histogram.Max(kvp => kvp.Value);
        var height = (int)Math.Ceiling(maxValue * heightScale);
        var bitmap = new Bitmap(width + marginLeft, height + marginBottom + marginTop);
        var gfx = Graphics.FromImage(bitmap);
        gfx.Clear(Color.White);
        var blue = new Pen(Color.Blue);
        foreach (var kvp in histogram)
        {
            var x = kvp.Key - min + marginLeft;
            gfx.DrawLine(blue, new PointF(x, (height - kvp.Value * heightScale) + marginTop), new PointF(x, height + marginTop));
        }
        
        var font = new Font("Arial", 10);
        StringFormat strF = new StringFormat();
        strF.Alignment = StringAlignment.Far;
        
        for (var i = (int)((maxValue - yLabelOffset) * heightScale); i >= 0; i -= 21)
        {
            gfx.DrawString(((int)(i / heightScale)).ToString(), font, Brushes.Black, marginLeft - 5, (maxValue * heightScale) - i - 6, strF);
        }
        
        for (var i = 0; i < width; i++)
        {
            if ((i + xLabelOffset) % 20 == 0)
            {
                var g = Graphics.FromImage(bitmap);
                g.TranslateTransform(i - 6 + marginLeft, height + 5 + marginTop);
                g.RotateTransform(270);
                g.DrawString((i + min).ToString(), font, Brushes.Black, 0, 0, strF);
            }
        }
        
        return bitmap;
    }
}