show derivative of the graph

[Petr M. <mi...@ph...>]

FYI: I've fixed that polar coordinates (distance between ruler and mouse 
cursor) are displayed also for log and semilog plots.

In addition, hitting hotkey '5' three times flips polar coordinates format 
among
    off / (distance,angle) / (distance,tangent)

(until now there were only the first two) => and that's how you can find
easily tangents of your curves:

- e.g. plot x*x
- put cursor on the curve and hit 'r'
- put cursor on the curve close to ruler and hit twice '5'

Hope this helps,
   PM