#674 fixed broken y2 rescaling in do_zoom_scroll_down()

[dima]

Currently the mouse-wheel-based scroll down is broken because of a typo. I'm attaching a patch to fix the typo and another patch to space out the code to make such typos more visible.

Simple bug demo:

1. plot this:

   set style data linespoints
   set grid
   set xtics
   set ytics nomirror
   set y2tics
   plot '-' notitle axes x1y2 , '-' notitle
   1 1
   2 4
   3 9
   4 16
   5 25
   e
   1 2
   2 4
   3 6
   4 8
   5 10
   e

2. use the mouse wheel to scroll up/down. Observe rescaling of y2 on the down strokes

[dima]

Silly sourceforge only lets me attach one file. Here's the second patch.