I'm using the identical of plotting commands to draw 3 individual plots on both a PC and Linux box and am not setting the terminal type. I have a plot size of1,.2 and 4 lines. On the linux box I get 2 columns of keys and on windows box I get 4 columns. It's very clear on the windows box that there is room for 2 or even 3 rows of keys, yet that are all drawn on a single row. In fact, if I set the height to .2001, I get 2 columns instead of 4.
Now here's something really silly. If I set the plot height to .24001, I see 2 columns on 2 of the plots and 1 column on the 3rd one. How could that be when they're all he same height?
Well, different platforms have different default terminals.
Different terminals have different default fonts, of different size.
Key layout depends on (what gnuplot believes to be) the size of strings in the graph.
So, by not specifying anything --- no terminal driver, no font, not size --- you've basically invited this to happen. If you don't want that, don't do it that way.