showslope does not work under recent versions of Matlab - sorry! 
 
showslope(N) drags a line in the current figure with a fixed slope N 
(does not work with docked windows; undock your window first!) 
 
Depending of the axis types, the 'slope' N means: 
     Y = N*X+A         for X linear and Y linear 
     Y = A*X^N         for X log and Y log 
     Y = A*EXP(N*X)    for X linear and Y log 
     Y = A+N*LOG(X)    for X log and Y linear (LOG = natural logarithm) 
 
The value N is displayed close to the line. N may also be the string of 
any valid Matlab expression (eg, 'pi', '22/7', ...), which will be 
displayed close to the line. showslope(N,'nolabel') does not display 
the value N in the figure. 
 
H = showslope(..) also returns a handle to the line. 
 
By default, the line is an 'annotation object', ie: it is attached to 
the window and not to the figure axes. As a consequence, the line may 
be further moved, or may be used for getslope. However, resizing 
the window may in some case shift the line, and turning the axes from 
lin to log turns a power law to a log law... To avoid these problems, 
specify showslope(N,'fix'), which makes the line a real plot object 
attached to the figure axes. 
 

getslope, getlineinfo, plotsample. 
Published output in the Help browser 
   showdemo showslope