2D ベクトル場の矢印を表示する.

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Sub Test_Quiver()
    Const M = 6, N = 6
    Dim xl(N - 1) As Double, yl(M - 1) As Double
    Dim X(M - 1, N - 1) As Double, Y(M - 1, N - 1) As Double
    Dim U(M - 1, N - 1) As Double, V(M - 1, N - 1) As Double
    Dim C(0, 0) As Long
    Dim I As Integer, J As Integer
    Dim Fig As Figure, Ax As Axs
    '-- Make data
    Call Linspace(-4, 4, N, xl())
    Call Linspace(-4, 4, M, yl())
    Call MeshGrid(N, M, xl(), yl(), X(), Y())
    For I = 0 To M - 1
        For J = 0 To N - 1
            U(I, J) = Y(I, J) + X(I, J)
            V(I, J) = Y(I, J) - X(I, J)
        Next
    Next
    '-- Plot
    Set Fig = Plt.Figure()
    Set Ax = Fig.Gca()
    Call Ax.Quiver(N, M, X(), Y(), U(), V(), C(), "color='C0', angles='xy', scale_units='xy', scale=5, width=.015")
    Call Plt.Show
End Sub