Recall an input from previous slide as a graphed variable

Students will select a principle amount to invest on the first slide. I want that value to carry over for the rest of the slides, as a variable on a graph.

So, they define P in the formula I=Prt on the first slide, then on the second they can set a value for “r” while the graph holds the previous P value

You can copy a numeric value from anywhere (except for a sketch, sadly) to anywhere (including sketch) by just creating a new variable.
i.e. if you want the graph on screen 4 (call it graph4) to include a value “n” from the graph on screen 1 (call it graph1), enter the following into the script for graph 4: number(“n”):graph1.number(“n”). The first number can even be named whatever you like as long as the source remains the same.