Simulated X & Y data for 100,000 rows

Here's how I would probably approach this:

1) Enter desired starting value for X in A2 and Y in B2, or generate them randomly using one of Excel's built in random number generators (RANDBETWEEN() or RAND())
2) In C2, generate a random number using RANDBETWEEN() that represents the change in X.
3) In D2, generate a random number using RAND() that represents the change in Y. Note that, since you appear to want these numbers to be something other than 0 to 1, you will need to note the formula used to get a random real between a and b (see help file https://support.office.com/en-us/art...0-021ea9f5be73 ).
4) In A3, enter =A2+C2. Copy to B3 and then copy down as far as needed.
5) Note that, somewhere in steps 3 and 4, you will need a small algorithm to make sure X and Y don't exceed the limits you have imposed on them. I am not sure what rule(s) you would want to use here. I could see something in the choices for a and b that dictates the limits of the random number generators in C and D, or perhaps including simple MIN() and MAX() functions in A and B that keep the numbers from exceeding their limits.

That's an overview of how I would probably do this. The hardest part is probably developing step 3 -- the exact rules and limits you want to put on the random number generators. At this point, I will assume you know better than I do what those rules and limits ought to be and can implement them, rather than me trying to guess what those rules and limits ought to be.

have a go with this

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you can set the bounds for X & Y as well as the # of rows required in the code; you could easily adjust the code to retrieve these values from cells on the worksheet if they were something that needed to change regularly.

for the second requirement I adjust the upper and lower bounds for X based on the output of Y. my adjustment is relatively crude and am sure there are better ways to keep these numbers aligned, but it should be pretty easy to adjust as you see fit.

I have uploaded a copy of the file which contains the code.

the other change I made to the file was to create a dynamic Named Range (see this) for the data called DataSet, this will adjust to the number of rows of data you have, you can then point your chart at that Named Range so that it adjusts to whatever number of rows you generate.

note: I set the # of rows to 1000 in the attached file to keep the file size down so the site would accept the upload, a chart with 100,000 points made the file size too large to upload!