如果您有一个特定的模型来拟合您的点，则可以在geom_smooth中使用method = nls。
就我从literature中了解到的情况而言，您尝试拟合的函数采用以下形式：

rosin_rammler <- function(x, A, B, C) {
  C * ((A / x) * (x / B)^A * exp(-((x / B)^(A - 1))))
}

我们可以直接在geom_smooth中使用它：

ggplot(allOnw, aes(x = xy + 1e-6, y = yy, col = sem1)) +
  geom_point() +
  geom_smooth(formula = y ~ rosin_rammler(x, A, B, C), method = nls,
              method.args = list(algorithm = "port",
                                 start = list(A = 5, B = 20, C = 1000), 
                                 lower = c(1, 1)), se = FALSE, n = 1000) +
  labs(col = "S", x = "MP", y = "GT")+
  theme(legend.position = "bottom")  +
  scale_x_continuous(breaks = 0:16 * 10, limits = c(0, 160)) +
  scale_color_manual(values = c("blue", "cyan", "darkgreen", 
                                      "darkorange", "gold", "darkorchid")) + 
  facet_wrap(vars(cat1), ncol = 1, strip.position = "right")