I'm trying to make a Scatchard plot using linear regression. How do I make the line extend far enough?

We don't recommend this method of creating a Scatchard plot (see this step-by-step description of an alternative procedure). But if you really want to do this, Prism's default method is to plot a linear regression line extending only over the range of your transformed input data. To change this, you'll need to run the linear regression once and note the value of the X intercept reported on the Results sheet. Then from the Results sheet, "Change...Analysis Parameters". In the output options section of the Parameters dialog, set "Start regression line.." to X = 0 and set "End regression line..." to X = the X intercept that you recorded before.