A past student made a good suggestion about making Excel graphs work for different lengths of time.

We set up a few additional columns (Col O to Col S) and cell reference
back to the original calculation of the Brownian motion simulation (Col B
to Col L). The "trick" we used in Col O to Col S is that we entered an IF
statement, and refer the cells in Col O to Col S back to time zero if the
time is bigger than the required steps. For example, say we input 250
steps, all the 251st to 500th cells would show the values at time zero.

Finally, we set the graph format by right clicking on the graph, select
"Chart type" option, choose "time scale" instead of "catagory" in the
X-axis format. That way the graph should have no problem adjusting to the
number of manual input steps.

Hope this method can be used to help the course.