# Panel data regression: plot and quadratic fitting line

I am conducting an OLS regression panel data analysis with package `PLM` in R.

I use the following script to obtain a plot and fitting line of variables D and GDPCAP:

``````yhat <-ols\$fitted
plot(mydata\$GDPCAP, mydata\$D, pch=19, xlab="GDPCAP", ylab="D")
abline(lm(mydata\$D~mydata\$G),lwd=3, col="red")
`````` Is there any script that can fit a quadratic line as well as provide the quadratic equation coefficients of the line?

The following script fits a quadratic line in the plot (in blue):

``````quadratic_model <- lm(D ~ GDPCAP + I(GDPCAP^2), data = mydata)
order_id <- order(mydata\$GDPCAP)
lines(x = mydata\$GDPCAP[order_id],
col = "blue",
lwd = 2)
`````` Coefficients of the quadratic equation are obtained as follows:

``````coeftest(quadratic_model, vcov. = vcovHC, type = "HC1")

t test of coefficients:

Estimate Std. Error t value  Pr(>|t|)
(Intercept) 49.970827   0.310524 160.924 < 2.2e-16 ***
GDPCAP      13.790826   0.340038  40.557 < 2.2e-16 ***
I(GDPCAP^2) -1.180603   0.070727 -16.692 < 2.2e-16 ***
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
``````

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