Dear pyx experts:

I'm curious if there is a way

> to specify the y2 axis as a function of the y axis? I am particularly

> interested in cases where the axes are not linearly related but are

> nonetheless monotonic. In the case that I am currently working on,

> the axes are related by y2 = y/(y+a) where a is a constant. Please

> let me know if this may be possible at any level of (in)elegance.

> to specify the y2 axis as a function of the y axis? I am particularly

> interested in cases where the axes are not linearly related but are

> nonetheless monotonic. In the case that I am currently working on,

> the axes are related by y2 = y/(y+a) where a is a constant. Please

> let me know if this may be possible at any level of (in)elegance.

On 08.07.05, Tom Kornack wrote:

> I often use y and y2 axes on my graphs. I'm curious if there is a way

> to specify the y2 axis as a function of the y axis? I am particularly

> interested in cases where the axes are not linearly related but are

> nonetheless monotonic. In the case that I am currently working on,

> the axes are related by y2 = y/(y+a) where a is a constant. Please

> let me know if this may be possible at any level of (in)elegance.

Well, its a quite complicated problem as soon as you do not fix the

axis ranges. The problem is, that for unfixed axis ranges, the

partitioning can have some influence on the axis range. So to make a

simple solution, we fix the axis ranges and go for it. Than you can do

the following (on top of PyX 0.7.1):

from pyx import *

class relatedaxis(object, graph.axis.lin):

def __init__(self, otheraxis, a, **kwargs):

self.otheraxis = otheraxis

self.a = a

graph.axis.lin.__init__(self, **kwargs)

def convert(self, value):

return self.otheraxis.convert(self.a*float(value)/(1.0-float(value)))

def invert(self, value):

otherinvert = self.otheraxis.invert(value)

return otherinvert/(otherinvert + self.a)

min = property(lambda self: self.invert(0), lambda self, min: None)

max = property(lambda self: self.invert(1), lambda self, max: None)

yaxis = graph.axis.lin(min=0, max=1)

y2axis = relatedaxis(yaxis, 1)

g = graph.graphxy(width=8, y=yaxis, y2=y2axis, x=graph.axis.lin(min=0, max=0.5))

g.plot(graph.data.function("y2=x"))

g.writeEPSfile("relatedaxis")

depend on the y axis. Another difficult thing is to get a proper axis

partitioning for this case. You can use the autolinear-parter (i.e.

the default), but I'm not sure whether equal distant ticks (in

y2-values) are the right thing for you. Unfortunately you can't use

the autologarithmic-parter (not just because we have a zero at the

axis in our example above, but also because the convert method always

fails for value=1). However, you can always place manual ticks and set

parter to None, or you can write your own (automatic) parter.