A new Manual Fit feature was introduced with version 6.0 of Graphical Analysis Pro, released in April 2024. A Pro subscription is needed to access the feature. 

With manual fitting, you can adjust a line superimposed on a graph, and the slope and intercept of the line in data units are displayed. Unlike the existing Curve Fit feature, Graphical Analysis does not optimize the fit. That is up to you. Up to three of these manual fit lines can be added to a graph, allowing an estimate of reasonable slope ranges. 

The RMSE (root mean square error) is calculated against the points plotted on the graph, providing a numerical measure of how close the manual fit is to the plotted points.

To use the manual fit, collect or enter some data, adjusting a graph to display the data as desired. You may want to scale the graph for a little more room to make it easy to adjust the manual lines. 

Click the Graph Options menu at lower left, and choose Add Manual Fit. 

A manual fit line will be inserted into the graph. You may want to move the information box to a more convenient location. 

Grab one of the handles on the line to move it. 

Adjust the line to fit as you like. 

Note that the slope and intercept of the manual fit line update as you move the line about. The RMSE is a measure of how far away the line is from the points, on average, in units of the Y axis. 

You can add up to three of these manual fit lines at a time. One use of three lines is to graphically estimate the best fit along with the lines of maximum and minimum slope that make sense with the data, allowing you to graphically estimate the uncertainty of the slope value.