1. Here take a Statistica scatterplot as an example (PT_POOR vs. PT_RURAL from Statistica example dataset Poverty.sta). The default fitting function produced is Y = 13.3106+0.1371*x

2. Place mouse cursor on the graph outside background, right click and then select "Graph Options...".

3. In the "Graph Options" dialog, select "Custom Function". Click on "Add new function".

4. The function definition text box will be active. Enter the custom function that you want to add to graph. For example: Y = 0.1371*X + 20. This function line would be noted as "Function: 1".

Click OK. The custom function line will be added to the graph. 

5. You can right click mouse in the graph, select "Graph Options..." and "Custom Function" as shown in step 2 & 3. Click on "Add new function" to add another custom function to the graph. For example, Y = 0.1371*X+6.61. This custom function will be noted as "Function: 2".

Click OK, the new custom function will be added to the graph.

6. If you want to delete any custom functions, in the "Graph Options" dialog, select the corresponding function number, like 1, 2, click on "Delete" button to delete the selected function from the graph. If you want to change the colors and line patterns for any custom functions, you can choose that specific function number and click on "Line | Pattern..." as highlighted in below figure to set the colors and patterns of the function line to display in the graph.