Plotting U-shaped curves using predict and twoway

I am trying to test for an inversed U-shaped patterns between R&D attainment discrepancy (which is the difference between the level of R&D exhibited by a focal organization i at time t-1 and the average industry R&D level at time t-2) and innovative performance at time t+1 (measured as the number of patents filed by the focal firm).
To test for a U-shaped between R&D attainment discrepancy and innovative performance, I used a random-effects negative binomial model (supported by a Hausman test) including the direct (significant and positive) and the squared term (significant and negative) of my IV.

To corroborate my findings I would like to graph the X-Y relationship over the relevant range of X, while holding other continuous variables at the sample means, and binary variables at 1. Is the following procedure correct?

* Random-effects negative binomial regression for innovative performance
xtnbreg F1Patent RDgap RDgapSquared RD1lag SPFnegative SPFpositive SL L1Altman L1GD IMPintensityPERCE RelDiv UnrelDiv L1LogtotAsset AGE IndGrowth RDav IndNIP IndPAT _Iyear*, re


*Generating Linear predicion for F1Patent after xtnbreg
predict F1Patenthat

*Graphing the results
twoway line F1Patenthat RDgap if _est_IPModel==1, sort //*(_est_IPModel==1 corresponds to e(sample))
twoway qfitci F1Patenthat GapBEST if _est_IPModel==1, sort

I think that I failed in holding the remaining covariates at their means and significant binary variables at 1.


Many thanks in advance for your always precious help,


Ambra