Streamlit plotting demo

First, we're going to start to learn how to make Streamlit apps by reproducing the plotting demo we saw before in the Streamlit demo, with a Python file that we've made ourselves. In order to do that, we will do the following:

1. Make a Python file where we will house all our Streamlit code.
2. Use the plotting code given in the demo.
3. Make small edits for practice.
4. Run our file locally.

Our first step is to create a folder called plotting_app, which will house our first example. The following code makes this folder when run in the terminal, changes our working directory to plotting_app, and creates an empty Python file we'll call plot_demo.py:

mkdir plotting_app
cd plotting_app
touch plot_demo.py

Now that we've made a file called plot_demo.py, open it with any text editor (if you don't have one already, I'm partial to VS Code (https://code.visualstudio.com/download). When you open it up, copy and paste the...