Re: [Matplotlib-users] Problem with semilog and low number

[John H. <jdh...@ac...>]

>>>>> "Jean-Baptiste" =3D=3D Jean-Baptiste Cazier <Jean-Baptiste.cazier@d=
ecode.is> writes:

    Jean-Baptiste> S=E6ll !  I am trying to plot very small number for
    Jean-Baptiste> the Y-axis on semilogy but they do not appear at
    Jean-Baptiste> all unless one of the value is higher Moreover the
    Jean-Baptiste> labels on the Y axis become 0 below 0.001

    >>> semilogy([1.0, 2.3, 3.3],[9.4e-05, 9.4e-05, 9.4e-05]) <-- does
    >>> not work
    Jean-Baptiste> [<matplotlib.lines.Line2D instance at 0x935255c>]
    >>> semilogy([1.0, 2.3, 3.3],[9.4e-04, 9.4e-05, 9.4e-05]) <---
    >>> work
    Jean-Baptiste> [<matplotlib.lines.Line2D instance at 0x940e964>]

S=E6ll Jean!

Thanks for this example.  The relevant code which handles autoscaling
is in matplotlib.axis.autoscale_view.  I wasn't handling the special
case where min=3Dmax for log scaling (though I do handle it for linear
scaling).

Try this replacement code for axis.py: and the functions decade_down
and decade_up and replace the autoscale_view function.

def decade_down(x):
    'floor x to the nearest lower decade'
    lx =3D math.floor(math.log10(x))
    return 10**lx

def decade_up(x):
    'ceil x to the nearest higher decade'
    lx =3D math.ceil(math.log10(x))
    return 10**lx

class Axis(Artist):

    def autoscale_view(self):
        'Try to choose the view limits intelligently'

        vmin, vmax =3D self.datalim.bounds()
        if self._scale=3D=3D'linear':

            if vmin=3D=3Dvmax:
                vmin-=3D1
                vmax+=3D1
            try:
                (exponent, remainder) =3D divmod(math.log10(vmax - vmin),=
 1)
            except OverflowError:
                print >>sys.stderr, 'Overflow error in autoscale', vmin, =
vmax
                return
            if remainder < 0.5:
                exponent -=3D 1
            scale =3D 10**(-exponent)
            vmin =3D math.floor(scale*vmin)/scale
            vmax =3D math.ceil(scale*vmax)/scale

            self.viewlim.set_bounds(vmin, vmax)
        elif self._scale=3D=3D'log':
            if vmin=3D=3Dvmax:
                vmin =3D decade_down(vmin)
                vmax =3D decade_up(vmax)
                self.viewlim.set_bounds(vmin, vmax)


Let me know how this works for you,
JDH