drawx — draw a readable x axis
drawx(xscale0,dilat,ref_nbinter,font_axis,y0,x0)
xscale0 = a (1xnobs) string vector, representing the scale
ref_nbinter = a number indicating the maximum numbers of chars that can be displayed
dilat = the dilatation factor (the width of a single value of the axis)
font_axis = size of characters on the axis
y0=value of the y axis where to draw the x axis
x0=value of the first x value used (default: 1)
nothing (printed on a graphic window)