Passing `log='y'` to slopegraph() throws warnings

```R
> slopegraph(t(df[1:2, taxa]), names.arg=c('sample1', 'sample2'), log='y')
There were 12 warnings (use warnings() to see them)
> warnings()
Warning messages:
1: In plot.xy(xy.coords(x, y), type = type, ...) :
  "log" is not a graphical parameter
2: In plot.xy(xy.coords(x, y), type = type, ...) :
  "log" is not a graphical parameter
```