`pssac` produces different polarity by Y-size in J-option

I have updated `gmt6` recently and found a very useful module `pssac` (`gmt sac` as a modern name). It reads sac files directly and make a record section while `pswiggle` needs an additional conversion from the sac file to xy-file. But, I like to notice that a feature of `pssac` can mislead `gmt` users. `pssac` represents the seismograms with different polarity when Y-size in J-option is positive and negative.

For example, "-J10c/5c" and "-J10c/-5c" make two reversed plots. This behavior is not natural because one expects that the upward direction of the paper is positive, in my opinion.

Let us make a sac file.

```
sac < a.ps
```
*Normal polarity (a.ps)*

```
gmt pssac seismogram.sac -R10/12/-1/1 -JX10c/-10c -P -M1 -Ba -Xc -Yc > b.ps
```
*Reversed polarity with the negative Y-size in J-option (b.ps)*

The similar module `pswiggle` does not behave like this way.

**Suggestion**
The polarity of the seismograms should be fixed as positive upward in the paper.