Cubic Function Graph Plotter

Dim a, b, c, d As Integer
Dim x As Double

Private Function f(x As Variant)
    ' Transform coordinates to start from center
    f = a * (5 - x) ^ 3 + b * (5 - x) ^ 2 + c * (5 - x) + d
End Function

Private Sub cmd_Clear_Click()
    pic_graph.Cls
    txt_a.Text = ""
    txt_b.Text = ""
    txt_c.Text = ""
    txt_d.Text = ""
End Sub

Private Sub cmd_draw_Click()
    Dim y As Double
    Dim w As Double
    
    a = Val(txt_a.Text)
    b = Val(txt_b.Text)
    c = Val(txt_c.Text)
    d = Val(txt_d.Text)
    
    ' Using a scale of 0.5 cm to represent 1 unit
    ' Transform coordinates as VB starts from top left
    For w = 0 To 10 Step 0.0001
        y = f(w)
        pic_graph.PSet (w, 5 - y)
    Next w
End Sub

Private a, b, c, d As Double

Private Function f(x As Double) As Double
    ' Calculate cubic function value
    Return a * Math.Pow(x, 3) + b * Math.Pow(x, 2) + c * x + d
End Function

Private Sub btnDraw_Click(sender As Object, e As EventArgs) Handles btnDraw.Click
    ' Parse input values
    If Not Double.TryParse(txtA.Text, a) OrElse Not Double.TryParse(txtB.Text, b) _
        OrElse Not Double.TryParse(txtC.Text, c) OrElse Not Double.TryParse(txtD.Text, d) Then
        MessageBox.Show("Please enter valid numeric coefficients")
        Return
    End If
    
    ' Create bitmap and graphics object
    Dim bmp As New Bitmap(picGraph.Width, picGraph.Height)
    Using g As Graphics = Graphics.FromImage(bmp)
        g.SmoothingMode = Drawing2D.SmoothingMode.AntiAlias
        g.Clear(Color.White)
        
        ' Draw axes
        Dim penAxis As New Pen(Color.Black, 1)
        Dim centerX As Integer = picGraph.Width \ 2
        Dim centerY As Integer = picGraph.Height \ 2
        g.DrawLine(penAxis, 0, centerY, picGraph.Width, centerY) ' X-axis
        g.DrawLine(penAxis, centerX, 0, centerX, picGraph.Height) ' Y-axis
        
        ' Draw graph
        Dim penGraph As New Pen(Color.Blue, 2)
        Dim scale As Double = 50 ' Pixels per unit
        Dim prevPoint As PointF? = Nothing
        
        ' Plot the function
        For x As Double = -5 To 5 Step 0.01
            Dim y As Double = f(x)
            Dim px As Single = centerX + x * scale
            Dim py As Single = centerY - y * scale
            
            If prevPoint IsNot Nothing Then
                g.DrawLine(penGraph, prevPoint.Value.X, prevPoint.Value.Y, px, py)
            End If
            prevPoint = New PointF(px, py)
        Next
    End Using
    
    ' Display the graph
    picGraph.Image = bmp
End Sub