Re: [Matplotlib-users] default tick behaviour

[matthew a. <ma...@ca...>]

Hi again,

I'm having more trouble with matplotlib ticks today. I wrote a little demo 
script that illustrates some of the problems:

...

#!/usr/bin/python

from matplotlib.matlab import *

xx = arange(0.002, 0.0101, 0.001)
print xx
# an instance of yy = rand(9), so all values are between 0 and 1
yy = [  5.94692328e-04,   1.62328354e-01,   7.56822907e-01,   2.28180047e-02,
        3.23820274e-01,   3.93120900e-01,   6.41332889e-01,   1.22474302e-02,
        5.03485402e-01]
subplot(211)
plot(xx, yy)
subplot(212)
plot(xx, yy)
autoaxis = axis()
print autoaxis
axis(autoaxis)
show()

...

* the x axis includes *two* 0.004 and *two* 0.008; this really worried me 
until I realised it was a cosmetic rounding / significant figures issue, 
however it's bad enough to be seriously misleading. I think the actual 
tick values are something like 0.0036 and 0.0044 but are both rounded to 
0.004.

* the data points lie *between* the x axis ticks, this is a side-effect of 
the above

* the poor choice of tick positions on the y axis -- they should be 
in round numbers like 0.2, 0.4, etc. The most significant varying figure 
should be a multiple of 1, 2, or 5.

* the tick labels should all have the same number of significant figures, 
e.g. 0.00, 0.15, 0.30, 0.45, 0.60, ... for the y axis

* after manually setting the axis (lower subplot), the last point is not 
plotted

I hope you find this feedback useful. I had a go at fixing it in axis.py,
but it's a) fiddly and b) I don't quite understand which part has
precendence when the axis changes during a zoom or pan. Getting the ticks
right depends on the correct bounds for the axis and the choice of
numticks. I noticed you have logic to clean up the bounds (vmin and vmax)
but not the ticklocs.

Thanks for matplotlib.

Cheers,
Matthew.