Example data tween_appear() Issue

structure(list(lat = c(50.16996, 50.16887, 50.16943, 50.17204, 
50.17247, 50.17227, 50.17302, 50.17354, 50.17365, 50.17249, 50.17205, 
50.17442, 50.17689, 50.1767, 50.17634, 50.17601, 50.17592, 50.17549, 
50.17402, 50.17379, 50.17314, 50.17323, 50.17566, 50.17714, 50.17919, 
50.18199, 50.18284, 50.1838, 50.18447, 50.18505, 50.1849, 50.18843, 
50.1909, 50.19203, 50.19184, 50.19247, 50.19406, 50.19381, 50.19416, 
50.1958, 50.19621, 50.19623, 50.19792, 50.19717, 50.1982, 50.19857, 
50.19901, 50.19888, 50.19992, 50.20095, 50.19956, 50.19857, 50.1974, 
50.19619, 50.19529, 50.19273, 50.19154, 50.1862, 50.18248, 50.18177, 
50.17728, 50.17513, 50.17383, 50.17108, 50.17116, 50.17228, 50.17224, 
50.17052, 50.1684, 50.16686, 50.16416, 50.16469, 50.16456, 50.16498, 
50.16516, 50.16487, 50.16593, 50.16568, 50.16353, 50.16055, 50.15625, 
50.14952, 50.14729, 50.1421, 50.13962, 50.13982, 50.13918, 50.13973, 
50.14071, 50.14282, 50.14149, 50.14156, 50.14854, 50.14923, 50.15176, 
50.15296, 50.15467, 50.15512, 50.15829, 50.15931, 50.1634, 50.16479, 
50.16582, 50.16655, 50.16751, 50.16865, 50.1709, 50.17169, 50.17321, 
50.17329, 50.17173, 50.16985), lon = c(-5.10796, -5.10558, -5.11005, 
-5.11234, -5.11533, -5.11673, -5.11736, -5.1189, -5.12262, -5.12619, 
-5.12895, -5.12951, -5.12883, -5.13092, -5.13119, -5.13298, -5.1365, 
-5.13821, -5.14077, -5.14261, -5.1442, -5.14585, -5.14973, -5.14675, 
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-5.1774, -5.17968, -5.17825, -5.1759, -5.17519, -5.17707, -5.17858, 
-5.17952, -5.18044, -5.18212, -5.18559, -5.18991, -5.19314, -5.19491, 
-5.19832, -5.19869, -5.20049, -5.20107, -5.2029, -5.20424, -5.20627, 
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-5.21889, -5.21984, -5.21932, -5.21191, -5.20561, -5.20323, -5.20065, 
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-5.18284, -5.17884, -5.17709, -5.17183, -5.16776, -5.16708, -5.16483, 
-5.1645, -5.16249, -5.1627, -5.15891, -5.15641, -5.15492, -5.151, 
-5.14783, -5.1459, -5.14604, -5.13846, -5.13429, -5.1261, -5.12399, 
-5.12214, -5.12052, -5.1151, -5.11195, -5.10437, -5.10307, -5.10142, 
-5.10034, -5.10069, -5.09775, -5.09813, -5.09981, -5.10428, -5.10801, 
-5.10944, -5.10999, -5.11065, -5.10814), grad = c(0, -1.79416716432266, 
6.22947142284364, -2.68443042681924, 3.9018643892292, 7.75251163374821, 
4.80489334762149, 10.5196929385563, 4.41743637059234, 5.65310911880992, 
4.62560849119362, 1.92107888731952, 1.28414190824996, 6.10640284240329, 
0.707920125020383, 6.3947836260868, 7.56468024481775, 4.44267987951931, 
0.950139988345945, 0.0129687969675702, -1.54291411756764, -0.218932773767188, 
-1.29435408851975, 3.50516255581757, 0.255461246832595, -4.47925260860736, 
-0.171948433053963, 0.880654787083903, -0.73742419520534, -0.239630971495108, 
-3.97708787400827, -0.115452016851018, 2.09441449931321, -2.30917626564311, 
-5.10064110611188, -1.27608664366021, 1.62339568580231, 0.397525758078687, 
3.11248660624225, 0.172198531816494, 2.67965419050329, 1.43219810980841, 
1.32288904937179, 1.79697650448743, -0.994004965891536, 4.97557131577067, 
1.58982270444949, 4.73007254678951, -2.09531236049125, 0.210868531844488, 
0.750958294404085, 0.843070954734238, 1.41619266566995, 0.380307969323808, 
4.58550170679513, -0.339221219535863, -5.79851471936256, 0.527126328082399, 
0.604572925750699, 2.97644518753188, 2.46201492730924, -2.10434202069813, 
-2.81864224135327, -0.934755927957575, -1.51522897425872, 0.0319702581932718, 
5.86654593279195, 7.3771547812686, 2.45176711885081, 1.01907173311832, 
-0.269012453642727, -4.4638754154951, -1.11846487639476, -1.48592367139579, 
2.20734610051828, 4.4719502302464, 0.326065323017083, -3.99789366134887, 
2.3503065730099, -0.0922816037095385, -3.05059223153774, -0.839367360598127, 
-1.03705806569478, -4.19418926954177, -2.34986649306904, -2.99752015743029, 
-4.38675316583567, -2.7221014830191, -0.704420738887379, 3.66384007495921, 
-1.17996461269136, 0.378546933649156, 0.0954789650961026, -1.99100589271702, 
-1.55625278718682, 2.86995798817086, 0.683954512233673, 1.14054076797346, 
-2.65885919568841, -4.77408947151168, -8.56452287142175, -12.5913681627085, 
-12.2651364068755, -1.89936068323951, -2.29481552646995, 0.628375388712469, 
1.87680678647283, 2.32855602537619, 4.43725490340526, -8.62117906633954, 
5.3198775106741, 2.54767559003415), n = 1:112), class = "data.frame", row.names = c(NA, 
-112L), .Names = c("lat", "lon", "grad", "n"))