R有光泽：当aes_string（）alternate不起作用时，如何让ggplot在其aes（）中使用变量

I am writing a shiny app and using ggplot to plot a density map. Unfortunately, I am not able to get ggplot to understand my input variable in the aes() section. Below is a part of the code that is problematic in my server.R:I am writing a shiny app and using ggplot to pl