\begin{Schunk}
\begin{Sinput}
 dat$x1b <- dat$x1 - mean(dat$x1, na.rm = TRUE)
 m4 <- lm(y ~ x1b, data = dat)
 plotSlopes(m4, plotx="x1b", interval="confidence", lwd = 0.4, axes=FALSE)
 axis(1)
 axis(2, pos = 0)
 abline(v = 0)
\end{Sinput}
\end{Schunk}
