plot.xbal `abs = TRUE` incorrect x bounds

```r
d <- data.frame(group = c(0,0,0,0,0,0,0,0,1,1,1),
                x1 = c(2,4,3,5,6,3,4,1,0,1,3),
                x2 = c(3,2,1,2,3,1,2,4,6,4,7))

xbal <- xBalance(group ~ x1 + x2, data = d, report = "all")

plot(xbal)
plot(xbal, abs = TRUE)
```

The first plot looks fine. The second (with `abs = TRUE`) has x-bounds at around 1.5 and 3.75, and completely loses the point for x1.