How to draw bitmaps in code

So, I had a task to create a graph in code

Not an easy task at you can see.  It took me about 12 hours to get it done from scratch.  My class takes the following parameters: 4 values – one for each color and bitmap size (width and height).

Here are some code samples:

Create a bitmap

Bitmap returnValue = new Bitmap(width, height);

Draw axes:

private static void DrawAxes(Bitmap result)
      Pen axesPen = new Pen(Color.Black, 1);
      Graphics worker = Graphics.FromImage(result);
      worker.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
      // draw diagonal axis lines
      worker.DrawLine(axesPen, GetTopLeftCorner(result), GetBottomRightCorner(result));

To draw red color polygon:

PointF[] coordinates = new PointF[] {
          GetRedCorner(result, red),
worker.DrawLine(graphPen, redYellowIntersect, GetRedCorner(result, red));
worker.DrawLine(graphPen, greenRedIntersect, GetRedCorner(result, red));
worker.FillPolygon(redBrush, coordinates);

One fun part was to write a code to determine where two lines intersect.  Here is entire function:

/// <summary>
    /// Find a point at which two lines intersect
    /// </summary>
    /// <param name="line1">First line</param>
    /// <param name="line2">Second line</param>
    /// <param name="middle">Coordinates of the middle of the bitmap</param>
    /// <param name="intersectionType">Does this intersect cross x or y axis</param>
    /// <returns></returns>
    private static PointF LinesIntersect(Line line1, Line line2, PointF middle, IntersectType intersectionType)
      PointF returnValue = new PointF();
      float a1, a2, b1, b2;
      if (line1.End.X == line1.Start.X)
        b1 = (line1.End.Y – line1.Start.Y);
        b1 = (line1.End.Y – line1.Start.Y) / (line1.End.X – line1.Start.X);

      if (line2.End.X == line2.Start.X)
        b2 = (line2.End.Y – line2.Start.Y);
        b2 = (line2.End.Y – line2.Start.Y) / (line2.End.X – line2.Start.X);

      a1 = line1.Start.Y – b1 * line1.Start.X;
      a2 = line2.Start.Y – b2 * line2.Start.X;
      returnValue.X = -(a1 – a2) / (b1 – b2);
      returnValue.Y = a1 + b1 * returnValue.X;

      switch (intersectionType)
        case IntersectType.Horizontal:
          returnValue.Y = middle.Y;
        case IntersectType.Vertical:
          returnValue.X = middle.X;
          throw new Exception("Invalid type");

      return returnValue;

Here are internal classes used in it:

/// <summary>
    /// Definition for a line - start and end points
    /// </summary>
    private struct Line
      public PointF Start;
      public PointF End;

      public Line(PointF start, PointF end)
        Start = start;
        End = end;

    /// <summary>
    /// Type of intersection - does cross y or x axis
    /// </summary>
    private enum IntersectType

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