Re: [Matplotlib-users] 'bug' in labelling axes for data with small variation?

[Hans F. <H.F...@so...>]

Hi John,

>    Hans> x=pylab.arange(0,1e-8,1e-9)+1.0 pylab.plot(x) pylab.show()
>
>    Hans> All works fine when I subtract the mean of x but there seems
>    Hans> to be a problem with labelling axes for plotted data which
>    Hans> is not close to zero but shows only small variations.
>
> I agree it's a bug.  It's not immediately clear to me what the labels
> should be though
>
>  1.0000000002
>  1.0000000004
>  1.0000000006
>
> and so on?  That takes up a lot of room.  Granted, correct but ugly
> is better than incorrect but pretty, but I'm curious if there is a
> better way to format these cases.  Perhaps ideal would be an indicator
> at the bottom or top of the y axis that read '1+' and then use 2e-9,
> 4e-9, etc as the actual tick labels.  Do you agree this is ideal?

That would be very cool ;-) [but not soo easy to code I suppose]. I just 
checked what Matlab does (seeing that they have been in the business for a 
while), and if I run the equivalent program (i.e. plotting date from 
1+0e-9, 1+1e-9, 1+2e-9,...,1e-8), the plot just shows "1" at each tick.

So: yes, I agree with your suggested ideal solution but as long as Matlab 
doesn't do this and there is a workaround (as you show below), this is 
probably not a high-priority bug ;-)

> To achieve this, as you may know, you can pass in a custom tick
> formatter.  Eg
>
>  import pylab
>
>  def myformatter(x, pos):
>      return '%1.0e'%(x-1)
>
>  ax = pylab.subplot(111)
>  x=pylab.arange(0,1e-8,1e-9)+1.0
>  ax.plot(x)
>  formatter = pylab.FuncFormatter(myformatter)
>  ax.yaxis.set_major_formatter(formatter)
>  ax.text(-.125, 1.025, '1+', transform=ax.transAxes)
>  pylab.show()
>
> See also examples/custom_ticker1.py.
>
> This is not to say that I don't want to fix this bug; I just wanted to
> make sure you are aware of this workaround.
That's very useful information -- I wasn't aware of this possibility and 
am sure it will be useful to others searching the mailing list for 
answers.

Best wishes,

Hans