semilogypn(...) is the same as SEMILOGY(...), except that the Y data 
may have positive and negative values. This is useful to detect 
unexpected negative values in the Y data (causing repeated 'Negative 
data ignored' warnings), or to visualize the magnitude of an 
oscillating signal in log scale. 
 
semilogypn(X,Y) plots X versus Y as logarithmic scales, where Y may 
have positive and negative values. By default, negative values are 
ploted with dashed lines and positive values with full lines. If X is 
not specified, Y is plotted versus its index. 
 
semilogypn(X,Y,LineSpecPos,LineSpecNeg) specifies LineSpecPos and 
LineSpecNeg for the line types, marker symbols and colors for the 
positive and negative values of Y. For instance, 
semilogypn(X,Y,'ro-','b*') plots the positive Y with red line and 'o' 
markers and the negative Y with blue stars '*'. 
 
semilogypn(X,Y,LineSpecPos,LineSpecNeg,'PropertyName',PropertyValue,...) 
sets property values for all lineseries graphics objects created by 
semilogypn. See the line reference page for more information. 
 
[HP, HN] = semilogypn(...) returns the handles to the two lineseries 
graphics objects. 
 

 
   x = linspace(1,10,200); 
   y = sin(x*2)./x; 
   semilogypn(x,y,'r.','bo'); 
   axis([1 10 1e-2 1]); 
 
   semilogypn(x,y,'b-','r:','LineWidth',2); 
 
   [hp, hn] = semilogypn(x,y,'k'); 
   set(hp, 'LineWidth', 1); 
   set(hn, 'LineWidth', 2); 
 

SEMILOGY, LOGLOG, loglogpn. 
Published output in the Help browser 
   showdemo semilogypn