Arch Linux

I can reproduce your problem and can add some more.

Also, when I do

> plot sin(x)

I get a warning:

warning: Warning - difficulty fitting plot titles into key

This is I guess because I use a tiling window manager (awesome) and the window with the plot is kind of constraint. This difficulty means, that the plot is not very well fitted in to the window (very small and not well visible).
Closing this window with "q" and hitting some random button in the gnuplot prompt will reopen the plotting window again with the same warning.

When I hit some key in the gnuplot prompt while the plooting window is still open, the plot is adjusted to the size of the plotting window. Closing this window with "q" and hitting some random button in the gnuplot prompt, does not lead to the behaviour described above, but will do just what is expected (that is letter appearing in prompt or whatever).

Can you reproduce this, too?

You have to select an area using the right mouse button to zoom in to that area. After this you can use the plus and minus buttons on top to swith back and forth between the previous zooming levels.

It works for me like this on 4.6.5, but also on 4.6.4. Re-open if it doesn't work you like that, complain upstream if you don't like the way it works ;-)