在堆叠的ggplot2图上设置自定义标签

Question

该问题是我上一个问题here的继续。

我有一个带有可用数据集的热图。数据集粘贴在下面：

library(ggplot2)
library(colorspace)
library(directlabels)

smalltest <- structure(list(x = c(-8, -7, -6, -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 
                                  -8, -7, -6, -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, -8, 
                                  -7, -6, -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, -8, -7, 
                                  -6, -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, -8, -7, -6, 
                                  -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, -8, -7, -6, -5, 
                                  -4, -3, -2, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8), 
                            y = c(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 
                                 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 
                                 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 
                                 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 
                                 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 5, 5, 5, 
                                 5, 5, 5, 5, 5, 5, 5, 5), 
                            z = c(0.353727812855041, 0.512450876310741, 
                                  0.668920303216554, 0.770942367746301, 0.829915628317595, 0.873001682466956, 
                                  0.900219155289838, 0.918353789896507, 0.936488424503176, 0.954641684205298, 
                                  0.961439844045867, 0.972770110446816, 0.975042372092157, 0.981846740297877, 
                                  0.986385055223408, 0.986385055223408, 0.986385055223408, 0.33104865495769, 
                                  0.464820298870698, 0.62128351741136, 0.752801524774481, 0.804964208774903, 
                                  0.850322524569605, 0.879812259037828, 0.913821683336127, 0.934222371222986, 
                                  0.950109577644919, 0.959173790765678, 0.970504057166626, 0.975042372092157, 
                                  0.981846740297877, 0.986385055223408, 0.986385055223408, 0.986385055223408, 
                                  0.31064175870568, 0.428544821292209, 0.589558771488704, 0.725596468681902, 
                                  0.786835782533385, 0.838986049803505, 0.872995474101805, 0.897946893644497, 
                                  0.920613634811545, 0.943299001074047, 0.956907737485488, 0.970504057166626, 
                                  0.970504057166626, 0.981840531932726, 0.986385055223408, 0.986385055223408, 
                                  0.986385055223408, 0.29023486245367, 0.419493024901753, 0.569145666871543, 
                                  0.702929727514853, 0.775480682671832, 0.827655783402557, 0.866197314261236, 
                                  0.891148733803927, 0.916075319886014, 0.931943901212494, 0.952369422559957, 
                                  0.970504057166626, 0.970504057166626, 0.981840531932726, 0.981840531932726, 
                                  0.986385055223408, 0.986385055223408, 0.272100227847001, 0.396807658639251, 
                                  0.557778150279687, 0.691580836018451, 0.766410261185922, 0.807248887150547, 
                                  0.857126892775325, 0.888876472158586, 0.911543213325635, 0.929684056297455, 
                                  0.941020531063555, 0.959155165670224, 0.968231795521285, 0.977302217007196, 
                                  0.981840531932726, 0.984112793578067, 0.984112793578067, 0.265302068006432, 
                                  0.396789033543797, 0.557784358644838, 0.680244361252351, 0.761871946260391, 
                                  0.800444518944826, 0.841264519813997, 0.882078312318017, 0.909277160045445, 
                                  0.931950109577645, 0.941014322698404, 0.954623059109845, 0.961421218950414, 
                                  0.972763902081665, 0.977302217007196, 0.984112793578067, 0.984112793578067)), 
                       row.names = c(NA, -102L), class = c("tbl_df", "tbl", "data.frame"))

我可以根据上述数据集生成热图和轮廓线。

    ggplot(smalltest, aes(x = x, y = y)) + 
      geom_tile(aes(fill = z)) + 
      scale_fill_continuous_divergingx(palette = 'RdBu', rev = FALSE, mid = 0.9, l3 = 0, p3 = 0.95, p4 = 0.85) + 
      scale_x_continuous(expand = c(0, 0), breaks = -8:8) + 
      scale_y_continuous(expand = c(0, 0), breaks = 0:5) + 
      geom_contour(aes(z = z), breaks = c(0.8, 0.9, 0.95), color = 'black', size = 1) +
      geom_dl(aes(label = c(rep(NA, 99), 'Low', 'Middle', 'High')), method = 'last.points')

我有以下问题：

1. 如何让geom_dl()仅查看/使用这些标签的轮廓线的末端或侧面？我最初添加rep(NA,99)是因为出现错误，指出label的长度必须等于1或数据集的长度（此处为102）。但是，我怎样才能仅通过自定义字符串向量呢？
2. 如何将轮廓线延伸到瓷砖的两端？

谢谢！

^{由reprex package（v0.3.0）于2019-11-07创建}