ggplot2：如何在对数刻度上绘制水平线

I have the folllowing dataframe:

我有以下数据框：

dput(AR.df)
structure(list(Type = structure(c(2L, 2L, 2L, 2L, 2L, 2L, 2L, 
2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 1L, 1L, 1L, 
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 
1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 
2L, 2L, 2L, 2L, 2L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 
2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 1L, 1L, 1L, 
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 
1L), .Label = c("Complex-valued", "Magnitude-only"), class = "factor"), 
    AR.coef = c(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 
    35, 40, 45, 50, 55, 60, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 
    20, 25, 30, 35, 40, 45, 50, 55, 60, 1, 2, 3, 4, 5, 6, 7, 
    8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 1, 2, 3, 
    4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 
    60, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 
    45, 50, 55, 60, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 
    30, 35, 40, 45, 50, 55, 60), variable = structure(c(1L, 1L, 
    1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 
    1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 
    1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 
    2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 
    2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 
    2L, 2L, 2L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 
    3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 
    3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L), .Label = c("p=0", 
    "p=1", "p=phat"), class = "factor"), value = c(0.07226, 0.08482, 
    0.08634, 0.0863, 0.08832, 0.08863, 0.08771, 0.08636, 0.08752, 
    0.08778, 0.08732, 0.08855, 0.08868, 0.08801, 0.08806, 0.08765, 
    0.08774, 0.08592, 0.0868, 0.08616, 0.08737, 0.09057, 0.08722, 
    0.08768, 0.08819, 0.08816, 0.08758, 0.08601, 0.08687, 0.08712, 
    0.08619, 0.08774, 0.08792, 0.08701, 0.08729, 0.08641, 0.08674, 
    0.08532, 0.08635, 0.08557, 0.05503, 0.05573, 0.05482, 0.0537, 
    0.05441, 0.05439, 0.05503, 0.05373, 0.05463, 0.0536, 0.05401, 
    0.05508, 0.05347, 0.05356, 0.05408, 0.05376, 0.05383, 0.05316, 
    0.0529, 0.05322, 0.05275, 0.05406, 0.05331, 0.05251, 0.05265, 
    0.0533, 0.05365, 0.0517, 0.05242, 0.05254, 0.05238, 0.0535, 
    0.05166, 0.05166, 0.05294, 0.0523, 0.05215, 0.05196, 0.05144, 
    0.05184, 0.06526, 0.06671, 0.06451, 0.06327, 0.06431, 0.06467, 
    0.06463, 0.06328, 0.0639, 0.06346, 0.06308, 0.06458, 0.06291, 
    0.063, 0.06351, 0.06288, 0.06372, 0.06227, 0.06239, 0.06268, 
    0.05505, 0.05666, 0.05595, 0.055, 0.05503, 0.05568, 0.05579, 
    0.05407, 0.05474, 0.05509, 0.05486, 0.05593, 0.05412, 0.0544, 
    0.05526, 0.05475, 0.05454, 0.0543, 0.05399, 0.05438)), row.names = c(NA, 
-120L), .Names = c("Type", "AR.coef", "variable", "value"), class = "data.frame")
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