I think what you really want to do is create a more complicated model that will allow you to see this plot. I will use the Cleansing data, to illustrate.
Choose Fit Model.
Specify Coal particles as the Y.
Select pH and Polymer and click Add.
Click Run.
The first graph you see is the picture that you want.
As for the slope of the lines, you can get that from the Parameter Estimates, but I think it is easier to see by going to the red triangle by Response Coal particles and select Estimates > Show prediction expression.
The common slope is 54.294. The estimates for each Polymer are the adjustment that needs to be made to the overall intercept.
Oh, and this is one model, so it minimizes the error across all three polymers and the pH.
Dan Obermiller