Let -Ra works for log graph.

If I use `-Ra` I could get values that are not compatible for a logarithmic graph as in the example below.

```
gmt begin test png
gmt math -T0/10240/1 T 10240 DIV 360 MUL 400 MUL COSD = t.txt
gmt spectrum1d t.txt -S256 -W --GMT_FFT=brenner -N -i1 > pow5.txt
gmt plot pow5.txt -Wgreen -Re -JX-15cl/4c -Bxa2f3 -Bya -Vi
gmt plot pow5.txt -Wgreen -Ra -JX-15cl/4c -Bxa2f3 -Bya -Vi -Yh+1c
gmt end #show
rm t.txt pow5.txt
```

For the -Re I got `-R2/256/1.44309083861e-16/85.3334737947`.
For the -Ra I got `-R0/260/-2/86` which is not compatible with a log graph.

I don't think it is very important and I don't need it (for now).

Reproduce the commands in the issue and compare the regions produced by -Re and -Ra for the logarithmic graph. Start at the GMT plot/region handling entry points used by the second plot command; done means -Ra produces bounds compatible with the requested logarithmic axis without breaking the shown example.

Written by the indexing model from the issue text.