if (present(enable)) then
    self%grid_enabled = enable
end if

if (present(axis)) then
    self%grid_axis = axis
    self%grid_enabled = .true.
end if

if (present(which)) then
    self%grid_which = which
    self%grid_enabled = .true.
end if

if (present(alpha)) then
    self%grid_alpha = alpha
    self%grid_enabled = .true.
end if

if (present(linestyle)) then
    self%grid_linestyle = linestyle
    self%grid_enabled = .true.
end if

if (present(color)) then
    self%grid_color = color
    self%grid_enabled = .true.
end if

subroutine draw_pdf_grid_lines(ctx, x_positions, y_positions, num_x_ticks, num_y_ticks, &
                             grid_axis, grid_which, grid_alpha, grid_linestyle, grid_color)
    !! Draw grid lines at tick positions for PDF backend
    type(pdf_context), intent(inout) :: ctx
    real(wp), intent(in) :: x_positions(:), y_positions(:)
    integer, intent(in) :: num_x_ticks, num_y_ticks
    character(len=*), intent(in), optional :: grid_axis, grid_which, grid_linestyle
    real(wp), intent(in), optional :: grid_alpha
    real(wp), intent(in), optional :: grid_color(3)

    character(len=10) :: axis_choice, which_choice
    real(wp) :: alpha_value, line_color(3)
    integer :: i
    real(wp) :: grid_y_top, grid_y_bottom, grid_x_left, grid_x_right
    character(len=100) :: draw_cmd

    ! Set default values
    axis_choice = 'both'
    which_choice = 'major'
    alpha_value = 0.3_wp
    line_color = [0.5_wp, 0.5_wp, 0.5_wp]

    if (present(grid_axis)) axis_choice = grid_axis
    if (present(grid_which)) which_choice = grid_which
    if (present(grid_alpha)) alpha_value = grid_alpha
    if (present(grid_color)) line_color = grid_color

    ! Calculate plot area boundaries (PDF coordinates: Y=0 at bottom)
    grid_y_bottom = real(ctx%height - ctx%plot_area%bottom - ctx%plot_area%height, wp)
    grid_y_top = real(ctx%height - ctx%plot_area%bottom, wp)
    grid_x_left = real(ctx%plot_area%left, wp)
    grid_x_right = real(ctx%plot_area%left + ctx%plot_area%width, wp)

    ! Set grid line color with transparency
    write(draw_cmd, '(F4.2, 1X, F4.2, 1X, F4.2, 1X, "RG")') line_color(1), line_color(2), line_color(3)
    call ctx%stream_writer%add_to_stream(draw_cmd)

    ! Draw vertical grid lines (at x tick positions)
    if (axis_choice == 'both' .or. axis_choice == 'x') then
        do i = 1, num_x_ticks
            ! Convert from raster to PDF coordinates
            write(draw_cmd, '(F8.2, 1X, F8.2, 1X, "m")') &
                x_positions(i), grid_y_bottom
            call ctx%stream_writer%add_to_stream(draw_cmd)
            write(draw_cmd, '(F8.2, 1X, F8.2, 1X, "l")') &
                x_positions(i), grid_y_top
            call ctx%stream_writer%add_to_stream(draw_cmd)
            call ctx%stream_writer%add_to_stream("S")
        end do
    end if

    ! Draw horizontal grid lines (at y tick positions)
    if (axis_choice == 'both' .or. axis_choice == 'y') then
        do i = 1, num_y_ticks
            ! Convert Y position to PDF coordinates
            write(draw_cmd, '(F8.2, 1X, F8.2, 1X, "m")') &
                grid_x_left, real(ctx%height, wp) - y_positions(i)
            call ctx%stream_writer%add_to_stream(draw_cmd)
            write(draw_cmd, '(F8.2, 1X, F8.2, 1X, "l")') &
                grid_x_right, real(ctx%height, wp) - y_positions(i)
            call ctx%stream_writer%add_to_stream(draw_cmd)
            call ctx%stream_writer%add_to_stream("S")
        end do
    end if
end subroutine draw_pdf_grid_lines

subroutine test_grid_basic()
    type(figure_t) :: fig
    real(wp) :: x(10), y(10)
    integer :: i

    call start_test("Basic grid lines")

    call fig%initialize(640, 480)

    ! Create test data
    do i = 1, 10
        x(i) = real(i, wp)
        y(i) = sin(real(i, wp) * 0.5_wp)
    end do

    call fig%add_plot(x, y)
    call fig%grid(.true.)
    call assert_true(fig%grid_enabled, "Grid should be enabled")

    call end_test()
end subroutine test_grid_basic

subroutine test_grid_major_only()
    type(figure_t) :: fig
    real(wp) :: x(5), y(5)
    integer :: i

    call start_test("Major grid lines only")

    call fig%initialize(640, 480)

    ! Create test data
    do i = 1, 5
        x(i) = real(i, wp)
        y(i) = real(i * 2, wp)
    end do

    call fig%add_plot(x, y)
    call fig%grid(which='major')
    call assert_true(fig%grid_enabled, "Grid should be enabled")
    call assert_equal_string(fig%grid_which, 'major', "Grid which should be major")

    call end_test()
end subroutine test_grid_major_only

subroutine test_grid_minor_only()
    type(figure_t) :: fig
    real(wp) :: x(5), y(5)
    integer :: i

    call start_test("Minor grid lines only")

    call fig%initialize(640, 480)

    ! Create test data
    do i = 1, 5
        x(i) = real(i, wp)
        y(i) = real(i * i, wp)
    end do

    call fig%add_plot(x, y)
    call fig%grid(which='minor')
    call assert_true(fig%grid_enabled, "Grid should be enabled")
    call assert_equal_string(fig%grid_which, 'minor', "Grid which should be minor")

    call end_test()
end subroutine test_grid_minor_only

subroutine test_grid_x_axis_only()
    type(figure_t) :: fig
    real(wp) :: x(3), y(3)
    integer :: i

    call start_test("X-axis grid lines only")

    call fig%initialize(640, 480)

    ! Create test data
    do i = 1, 3
        x(i) = real(i, wp)
        y(i) = real(i + 1, wp)
    end do

    call fig%add_plot(x, y)
    call fig%grid(axis='x')
    call assert_true(fig%grid_enabled, "Grid should be enabled")
    call assert_equal_string(fig%grid_axis, 'x', "Grid axis should be x")

    call end_test()
end subroutine test_grid_x_axis_only

subroutine test_grid_y_axis_only()
    type(figure_t) :: fig
    real(wp) :: x(3), y(3)
    integer :: i

    call start_test("Y-axis grid lines only")

    call fig%initialize(640, 480)

    ! Create test data
    do i = 1, 3
        x(i) = real(i, wp)
        y(i) = real(i * 3, wp)
    end do

    call fig%add_plot(x, y)
    call fig%grid(axis='y')
    call assert_true(fig%grid_enabled, "Grid should be enabled")
    call assert_equal_string(fig%grid_axis, 'y', "Grid axis should be y")

    call end_test()
end subroutine test_grid_y_axis_only

subroutine test_grid_customization()
    type(figure_t) :: fig
    real(wp) :: x(4), y(4)
    integer :: i

    call start_test("Grid customization")

    call fig%initialize(640, 480)

    ! Create test data
    do i = 1, 4
        x(i) = real(i, wp)
        y(i) = cos(real(i, wp))
    end do

    call fig%add_plot(x, y)
    call fig%grid(alpha=0.5_wp, linestyle='--')
    call assert_true(fig%grid_enabled, "Grid should be enabled")
    call assert_equal(real(fig%grid_alpha, wp), 0.5_wp, "Grid alpha")
    call assert_equal_string(fig%grid_linestyle, '--', "Grid linestyle")

    call end_test()
end subroutine test_grid_customization

subroutine test_grid_toggle()
    type(figure_t) :: fig
    real(wp) :: x(3), y(3)
    integer :: i

    call start_test("Grid toggle on/off")

    call fig%initialize(640, 480)

    ! Create test data
    do i = 1, 3
        x(i) = real(i, wp)
        y(i) = real(i, wp)
    end do

    call fig%add_plot(x, y)

    ! Turn grid on
    call fig%grid(.true.)
    call assert_true(fig%grid_enabled, "Grid should be enabled")

    ! Turn grid off
    call fig%grid(.false.)
    call assert_false(fig%grid_enabled, "Grid should be disabled")

    call end_test()
end subroutine test_grid_toggle