docs: def linestyle(i,a=5,b=3):
      '''
      provide one out of 25 unique combinations of style, color and mark
  
      use in combination with markevery=a+mod(i,b) to add spaced points,
      here a would be the base spacing that would depend on the data
      density, modulated with the number of lines to be plotted (b)
  
      Parameters
      ----------
      i : integer
          Number of linestyle combination - there are many....
      a : integer
          Spacing of marks.  The default is 5.
      b : integer
          Modulation in case of plotting many nearby lines.  The default
          is 3.
  
      Examples
      --------
  
      >>> plot(x,sin(x),linestyle(7)[0], markevery=linestyle(7)[1])
  
  
      (c) 2014 FH
      '''
  
      lines=['-','--','-.',':']
      points=['v','^','<','>','1','2','3','4','s','p','*','h','H','+','x','D','d','o']
      colors=['b','g','r','c','m','k']
      ls_string = colors[sc.mod(i,6)]+lines[sc.mod(i,4)]+points[sc.mod(i,18)]
      mark_i    = a+sc.mod(i,b)
      return ls_string,int(mark_i)