doesn't draw a line on full area of a custom control after resize it, C#

using System.Drawing;
using System.Windows.Forms;

namespace SevenSegmentDigit
{
    public partial class DigitDisplay : Label
    {
        Graphics grx;
        Pen roundPen;
        public DigitDisplay()
        {
            InitializeComponent();            
            SetAutoSizeMode(AutoSizeMode.GrowAndShrink);
            this.ResizeRedraw = true;
            AutoSize = false;
            grx = this.CreateGraphics();
            roundPen = new Pen(Brushes.Black, 2);
            roundPen.StartCap = System.Drawing.Drawing2D.LineCap.Round;
            roundPen.EndCap = System.Drawing.Drawing2D.LineCap.Round;                       
        }
        protected override void OnPaint(PaintEventArgs pe)
        {
            showDigit();
            base.OnPaint(pe);            
        }
        protected override void OnSizeChanged(System.EventArgs e)
        {                       
            showDigit();
            Invalidate(); 
        }
        protected override void OnResize(System.EventArgs e)
        {                       
            showDigit();
            Invalidate();
        }                
        
        private void showDigit()
        {
            grx.Clear(this.BackColor);
            switch (digitValue)
            {
                case 1:
                    drawOne();
                    break;
                case 2:
                    drawTwo();
                    break;
            };
        }
        private int digitValue = 0;
        public int DisplayDigit
        {
            get { return digitValue; }
            set { digitValue = value; showDigit(); }
        }
        private void drawOne()
        {
            grx.DrawLine(roundPen, 30, 10, 30, 24);
            grx.DrawLine(roundPen, 30, 26, 30, 40);
        }


        private void drawTwo()
        {
            grx.DrawLine(roundPen, 14, 10, 29, 10);
            grx.DrawLine(roundPen, 30, 10, 30, 24);
            grx.DrawLine(roundPen, 14, 25, 29, 25);
            grx.DrawLine(roundPen, 14, 26, 14, 40);
            grx.DrawLine(roundPen, 14, 41, 29, 41);
        }
}
}