Shiny App needs to create new selectInputs for each file uploaded

TL;DR I’m learning to code with R. I have a Shiny app I’m developing that would produce a ggplot histogram from a series of Excel sheets the user uploads. The problem is getting a dropdown menu with a list of colnames so the user can choose which columns in the spreadsheet to use for the histogram. Please see below and let me know how I can make this work. Apologies if this code is messy; it’s my very first Shiny app.

Click here for the code of my Shiny App

Abstract: I am doing a volunteer project for a state-level authority in the US. They organize the efforts of most non-profit organizations in the state and offer grants, etc. As part of this, they conduct surveys using Qualtrics, and then get their results in Excel sheets. I am developing a web app they can use to analyze the survey results from various years and compare them in a dodged bar plot. The surveys change from year to year, but we are looking for the same information from each year: name of the organization, the number of volunteers or volunteer hours logged, and the year in which the survey was conducted.

At this point, I want the app to request the number of files to be uploaded (there could be multiples from the same year or even more than just 2 years of data on the plot). Then, based on the number the user chooses, to generate a separate fileInput spot for each file. As each file is uploaded, I want the app to take a list of the column names using colnames after reading the Excel file using read_excel. I want three drop-down menus to appear after that command runs, each with the list of column names: one for the organization, one for the number of volunteers/hours, and one for the year of the survey. From there, I want those three inputs to be used to analyze and combine the data from each year and put it into a ggplot command. The resulting plot should have the organizations listed in the x axis, the number of volunteers/hours on the y axis, and the color of each bar change for each year, the years being dodged, that is, placed side-by-side corresponding to each year. The ggplot will take inputs for the title, axis labels, and the name under which to save a PDF of the graph.

Though I have it in my head, putting it down into code is the hard part. Any help you can offer, even just getting a fresh pair of eyes on the code, would be greatly appreciated. I have included the code below for you or anyone who can help to look over and edit. Many thanks in advance; best wishes.

Fearless-Loess

Click here for the code of my Shiny App