In the previous version of the graphing tool, there was a button for linear regression. That has been replaced by Curve Fits.
To add a linear regression to a graph:
Click the cog at the upper right of the graph.
Select Curve Fit.
Note: You must have data on both axes to add a Curve Fit.
You can click on data points on the graph to include/exclude them from the curve fit calculations.
As a student, your data always saves as you go so you can return to your work later to continue or revise.