The "item store" that PROC PLM reads does not contain observation information, just a summary of the model. So you can't overlay the fit on a scatter plot with PROC PLM. However, you can use the OUTPUT statement to save out the predictions and then use PROC SGPLOT to visualize the model. See the example code in
https://communities.sas.com/t5/SAS-Statistical-Procedures/How-to-plot-regression-line-from-mixed-models-in-my-observed/m-p/393750#M20565
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