Changing the slope of a chart when user change on the sliding scale

It's hard to say, though I am generally optimistic that most things are possible with enough time, knowledge, effort, and ingenuity.

At this point, I am having a hard time visualizing what you mean by "elastic/inelastic" and "sliding scale", and how a user will interact with these elements. It might also help to know what chart type you are using. Something that involves "slope" suggests a line or scatter chart to me. I also find that a lot of chart questions end up being spreadsheet questions, and I wonder if this is likewise going to be about what to do in the spreadsheet to get the chart to change.

With more details about what you are trying to do, we can see how we can help.

So I see a few steps to this:

1) Enter the slope (or whatever your desired input data are) -- you mention using a slider. How much of your question is how to add a slider or similar control to a spreadsheet? https://support.office.com/en-us/art...rs=en-US&ad=US I don't use these kinds of controls, so, other than being aware they exist, I'm not sure how much I can help with using them.
2) From the slope and the desired placement on the graph, compute the desired x and y values for the end points of the line segment in the spreadsheet. This is probably more algebra than spreadsheet. Do you recall how to generate a line based on slope?
3) Generate the chart. (of course, this only needs to be done when first building the spreadsheet. Once the chart is in place, it should update as data are changed/calculated.)

Does it look like I am understanding what you are trying to do? Which of those steps do you have trouble with?

If I understand the question, it sounds like you are stuck on step 2 -- how to take the angle or slope (depending on your preference) that the slider control gives and calculate two x,y data points for the chart.

You talk about having the slider give an angle in degrees. That might be the easiest way to do this -- assuming you remember your basic trig and can remember what you learned about the unit circle (I have a spreadsheet here, if it will help: https://www.excelforum.com/tips-and-...han-excel.html ).
1) Enter 0 and 0 in two adjacent cells to represent one end of the line.
2) Enter =COS(RADIANS(cell with slider output)) beneath the "x" 0.
3) Enter =SIN(RADIANS(cell with slider output)) beneath the "y" 0.
4) Create scatter chart using these 4 cells, make sure that it assigns the correct "series in rows/columns", and possible format the axes to be 0 to 1.

If you find the trig confusing, the same thing can be done by having the slider output a "rise over run" slope and then remember your early algebra (http://www.purplemath.com/modules/graphing.htm ) about graphing and select reasonable values the x and y values for the two data points, and chart them. (I would probably use 0,0 for one point again, then use x2=1 y2=cell with slope*1 to get the second y point.). This approach would definitely require that you limit the values of the axes to something reasonable (0 to 1 again?).

The key to tying the slider to the chart is to use the output of the slider to compute the x,y values for the data points that will be charted in the chart.