The :label_rotation attribute can be set to a value between [0,90] (in degrees

    g = Gruff::Line.new
    g.title = "Labels rotated 90 degrees"
    g.labels = {
      0 => '5/6', 
      1 => '5/15', 
      2 => '5/24', 
      3 => '5/30', 
    }
    g.data(:apples, [-1, 0, 4, -4])
    g.data(:peaches, [10, 8, 6, 3])
    g.hide_line_markers = false
    g.label_rotation = 90
    g.x_axis_label = "The x-axis labels are rotated, see?"

Result:

It will adjust the spacing reserved for the labels to account for the height of the rotated text.