ggplot中如何独立设置折线与点的大小、颜色、线型等样式？

你遇到的冲突是因为geom_line和geom_point同时将size美学映射到了Genotype变量，ggplot默认对同一种美学的映射共用同一个缩放规则，所以散点的大小设置会同步影响折线。
下面提供两种实现独立设置的方案：

方案1：无需额外依赖包，手动拆分折线图层 #

适合分组较少的场景，直接为不同基因型分别绘制折线，不需要做美学映射，自然不会和散点的缩放规则冲突。
修改后的完整代码：

# 加载包
library(ggplot2)

# 数据集代码
Genotype<-rep(c("GenotypeA","GenotypeB"),each=6)
Nitrogen<- rep(c(0,20,50,100,150,200),2)
Yield<- c(25,27,26,27,30,35,10,12,14,16,18,20)
dataA<- data.frame(Genotype,Nitrogen,Yield)

# 绘图代码
ggplot(data=dataA, aes(x=Nitrogen, y=Yield, fill=Genotype, shape=Genotype))+
  # 单独绘制两种基因型的折线，直接指定样式，不做映射
  geom_line(data = subset(dataA, Genotype == "GenotypeA"), 
            size = 1, color = "Green", linetype = "solid") +
  geom_line(data = subset(dataA, Genotype == "GenotypeB"), 
            size = 2, color = "Blue", linetype = "dashed") +
  # 散点图层和原有逻辑一致
  geom_point(aes(size=Genotype)) +
  scale_shape_manual(values= c(21,22)) +
  scale_fill_manual(values= c("White", "Orange")) +
  scale_size_manual(values=c(9,6))+
  scale_x_continuous(breaks = seq(0,200,50),limits = c(0,200)) +
  scale_y_continuous(breaks = seq(0,40,5),limits = c(0,40)) +
  labs(x="Nitrogen", y="Yield") +
  theme(axis.title.x = element_text (face = "plain", size = 15, color = "black"),
        axis.title.y = element_text (face = "plain", size = 15, color = "black"),
        axis.text.x = element_text(size= 15, margin = margin(t=0, r=0, b=10, l=0)),
        axis.text.y = element_text(size= 15, margin = margin(t=0, r=0, b=0, l=10)),
        axis.line = element_line(size = 0.5, colour = "black"),
        legend.position = 'bottom',
        legend.key = element_rect(color = "white", fill = "white"), 
        legend.key.size = unit(0.5,"cm"),
        legend.title = element_text(face = "plain", size = 14, color = "Black"),
        legend.text = element_text(face = "plain", size = 14, color = "Black"),
        plot.margin=unit(c(0,0,0,0),"cm")) +
  windows(width=5.5, height=5)

方案2：使用ggnewscale包创建独立缩放规则 #

适合分组较多的场景，通过新增独立的size缩放规则实现样式分离，代码更简洁易维护。
需要先安装加载依赖包：

install.packages("ggnewscale")
library(ggnewscale)

修改后的完整绘图代码：

ggplot(data=dataA, aes(x=Nitrogen, y=Yield))+
  # 先绘制折线图层，映射相关美学
  geom_line(aes(color = Genotype, linetype = Genotype, size = Genotype, group = Genotype)) +
  # 设置折线的样式规则，统一name保证图例合并
  scale_color_manual(name = "Genotype", values = c("GenotypeA" = "Green", "GenotypeB" = "Blue")) +
  scale_linetype_manual(name = "Genotype", values = c("GenotypeA" = "solid", "GenotypeB" = "dashed")) +
  scale_size_manual(name = "Genotype", values = c("GenotypeA" = 1, "GenotypeB" = 2)) +
  # 新增独立的size缩放规则，后续的size映射和前面折线的size完全独立
  new_scale("size") +
  # 绘制散点图层，映射散点的专属美学
  geom_point(aes(fill = Genotype, shape = Genotype, size = Genotype)) +
  # 设置散点的样式规则，name和折线保持一致，图例自动合并
  scale_shape_manual(name = "Genotype", values= c(21,22)) +
  scale_fill_manual(name = "Genotype", values= c("White", "Orange")) +
  scale_size_manual(name = "Genotype", values=c(9,6))+
  # 后续坐标轴和主题设置不变
  scale_x_continuous(breaks = seq(0,200,50),limits = c(0,200)) +
  scale_y_continuous(breaks = seq(0,40,5),limits = c(0,40)) +
  labs(x="Nitrogen", y="Yield") +
  theme(axis.title.x = element_text (face = "plain", size = 15, color = "black"),
        axis.title.y = element_text (face = "plain", size = 15, color = "black"),
        axis.text.x = element_text(size= 15, margin = margin(t=0, r=0, b=10, l=0)),
        axis.text.y = element_text(size= 15, margin = margin(t=0, r=0, b=0, l=10)),
        axis.line = element_line(size = 0.5, colour = "black"),
        legend.position = 'bottom',
        legend.key = element_rect(color = "white", fill = "white"), 
        legend.key.size = unit(0.5,"cm"),
        legend.title = element_text(face = "plain", size = 14, color = "Black"),
        legend.text = element_text(face = "plain", size = 14, color = "Black"),
        plot.margin=unit(c(0,0,0,0),"cm")) +
  windows(width=5.5, height=5)

内容的提问来源于stack exchange，提问作者J.K Kim