Specifying log axis annotation/gridline spacing that is not a power of 10

Consider the following code snippet:

gmt psbasemap -R-50/50/200/1030 -JX25.1/-17l -Bpx10 -Bpy3g3+l"pressure (hPa)" -Bsyg2 -Bpxa10g10+l"temperature (\217C)" -Bsxg40 -BSW -P > $output

With that, I can create the figure shown below. This is basically what I want, but I would like to place y-axis gridlines every 50 hPa instead of the 100 I get with the -Bpy3g3 annotation. For the life of me, I cannot figure out how to specify these additional gridlines inside the -B flag. Is the only way to achieve this to plot additional lines manually with psxy?

Thanks for the help.

I think you will have to do this via custom annotations and list the ones you want, i.e., 1, 5, 10, 50, …
I have not seen this type of annotation/gridlines as a standard thing, but if you can convince me that this is used all the time in some fields it would not be hard for us to add another code. Perhaps “5” could mean 1-5-10-50, etc in that case.

Thanks Paul, custom annotations have served my purpose for now. But, it is a little tricky. Custom annotation apparently cannot be combined with standard annotations in the same psbasemap call, so creating the image below relies on a number of successive calls to psbasemap.

By the way, this basemap will be for plotting atmospheric soundings (I will use it for teaching meteorology). Ultimately, I will come back to you with a separate question about how I might make a custom map projection to skew the y-axis for a Skew-T diagram.